POP: a musical of general appeal, especially among young people that originated as a distinct genre in the 1950s. It is generally characterised by a heavy rhythmic element and the use of electrical amplification. - Collins English Dictionary.

There was a time when, Bono will freely confess, he thought the word "pop" was a term of abuse. "It seemed sort of insulting and lightweight," he says. But lately, as anyone who has been paying attention will be aware, he has rather warmed to the notion of being a pop star, in a pop group, making pop music. For "pop", he has decided, is a word full of freedom, a term that can mean almost anything, with none of the leaden, old-fashioned connotations of rock. To demonstrate this, he will drag out an enormous dictionary and recite a sprawling, multiplicitous definition of Pop to anyone who'll listen.

"Let me see now . . . Pox," he laughs, flicking through the pages. "Practise. Pram. Prant. Prang. God, all my words are here! Pratfall. Preach. Prayer rug. Hold on, I'm past it. Here we go. P-O . . . Pop. Pop, poppy, pop. (1) To make or cause to make a light, sharp explosive sound. Isn't that great? (2) To burst open, with such a sound. (3) To come or to go rapidly or suddenly. (4) To protrude: 'her eyes popped with amazement'. (5) To shoot at a target with firearms. (6) To place with sudden movement: 'she popped some tablets into her mouth'. (7) To pawn: 'he popped his watch yesterday'. (8) To take a drug in pill form or as an injection. (9) To pop the question, to propose marriage. (10) A light sharp explosive sound, like crack. (11) A flavoured non-alcoholic carbonated beverage - actually I like that one best. And then it's also down as 'an informal word for father'. I like that. I like that a lot."

He idly continues his preamble through the dictionary. "Popcorn. Pope. That's pop after someone's taken an E!" he laughs. "Pop John Paul III!"

Noel Gallagher, another figure who could be said to stand astride the worlds of both pop and rock, legs akimbo, with a cuban heel in either camp and a guitar clutched in both hands, has his own, rather unique definition of Pop. He came up with it at four in the morning, in the almost deserted Tosca Cafe in San Francisco. U2 and Oasis had just played the first of two dates in Oakland stadium and a small group of diehards, fired up on alcohol and adrenaline, were still toasting their success.

Bono, who had just dropped a coin into the opera juke box, was standing astride the bar delivering a magnificent, drunken rendition of 'O Sole Mio'. Liam Gallagher was perched precariously on a barstool beneath him, a grin of disbelief pasted across his face. Noel Gallagher, leaning against a wall, bottle of beer in hand, eyes half-closed, smiled with the satisfaction of the cat who got the cream. "You know what POP stands for?" he joked. "Paddies on the Piss!"

There are those in the British press who would probably suggest other acronyms for the word Pop, like Pretentious Overblown Pastiche or Posers Out to Pasture. Elements of the British media - along with a majority of British rock bands - have long displayed a sneering and supercilious attitude to the Irish group. During the last few weeks, newspapers have seized on reports of cancelled shows with undisguised glee - backed by the kind of research that gives journalism a bad name. Conveniently glossing over legitimate reasons for the cancellation of two U2 dates, ignoring the addition of extra shows due to increased demand and finding no room in their stories for the fact that receipts of $130 million have already made this U2's most successful tour ever, The Guardian, Independent, Daily Star, Daily Express and Observer have (mis)informed their readers that PopMart is becoming FlopMart.

"If hubris has a sound" scoffed the Observer, "it is the hiss of air leaking out of the giant inflatable olive that is key prop on the world's most expensive rock tour." "U2 are feeling the cool wind of rejection for the first time," gloated the Guardian.

Finding these apparent underachievers playing to 50,000 admirers with Oasis as their support band, it seems safe to conclude that reports of the demise of U2 have been greatly exaggerated. Surveying the elegant circular stadium from a glass-walled office high up behind the stage, U2's manager Paul McGuinness is in bullish form. "We're two months into a world tour and we've already sold two million tickets," he declares. "And I confidently expect to sell over five million before we finish next year."

Dressed in a white boxing robe, like a fighter preparing to defend his title, Bono wanders agitatedly around the dressing room before the show. "I just wonder why they don't want us to win?" he says of his critics in the British media. "I feel it's an old public school thing. We're the outsiders being dragged through the bushes. We're the ruddy Irish boys getting a kicking."

Before you start speculating that U2's frontman is entering an advanced stage of superstar paranoia, I should mention that he makes these observations with a provocative grin playing on his lips. He is, in fact, philosophical about the negative press. "The term stadium rock is a term of abuse," he remarks, "but it's a bogus term. It's a bit like pub rock, which is a term of abuse for bands who won't ever play outside of pubs and generally play the blues - badly - and drink a lot of beer. But not all groups that play in pubs are pub rockers. We can play anywhere we want. It just happens that we have the magic wand to turn these large open spaces into something else."

He's on a roll now, bobbing and weaving like a genuine world heavyweight.

"I think the media have a problem with respecting 50,000 people. They have a problem with popularity and mass appeal. Well, I come from the position that 50,000 people gathered together might not be wrong." He laughs at his paraphrasing of the legendary Elvis Presley publicity line. "In fact, if they're gathered together at one of our gigs, I'm tempted to think they might be right!"

Oasis take the stage at dusk. In their own country, the boys of Britpop have played to the largest audiences ever gathered in one place, but here in America they are a sideshow rather than the main event. The stadium is still filling with U2's followers as the five-piece, augmented by a keyboard player, blast through a rocking, no nonsense set. It is the first real gig they have played in 10 months, and they clearly revel in this opportunity to strut their stuff.

Liam's singing is impossibly balanced between passion and nonchalance, while Noel's incendiary lead guitar cuts through the crowd's indifference, bringing people to their feet for a climactic, cataclysmic 'Champagne Supernova'. And astonishingly, Oasis, so used to being revered on their own terms, play the part of understudies with grace and humility, thanking U2 and the audience for the opportunity to perform. "If me mam could see me now," crows Liam, "She'd say, 'You done good, lad, you done good'."

"I love stadium gigs," Noel declares afterwards, backstage, eyes burning with excitement. "There's just so many people. How many bands can do this? How many?" He looks defiant, as he proposes a topsy-turvy theory of their status in the rock world. "U2 and Oasis are the underground and everybody else is the mainstream. Cause they're all afraid to be big. They're afraid of success!"

U2's manager declares himself suitably impressed with the leading challengers for the unofficial title of Greatest Rock Group In The World. "You can't help but admire their appetite and style," says McGuinness. He nonetheless holds reservations about how Oasis might take their live show to the next level. "Not many groups can play stadiums. You have to embrace the size, which requires a degree of theatrics. For some reason, a whole generation of groups have turned their back on the theatrical side of rock, which I find disappointing."

It is not an accusation that can be levelled at his own charges. Rather, they have gone beyond theatre. The PopMart show is genuinely spectacular, appealing on every conceivable level: artistic, intellectual, emotional, visual and musical. The band play beneath an arch of glittering neon, before an enormous video wall alive with inventive pop art imagery. A guitar solo is delivered to a mind-bending psychedelic display, spread across 700 square metres of screen. The group arrive for encores inside their own glitterball UFO. The show has constant momentum and is delivered on a scale that makes the full moon, suspended in a cloudless sky, look like just another part of the lighting rig.

Yet, most impressively of all, there is room for personality, improvisation and intimacy. U2 transcend the problem of physical distance with the generosity of their performance. Bono reaches out to audiences with the exposed humanity of a genuinely great stage performer - albeit one who is employing all the resources the modern world can provide. In an attempt to encapsulate this mixture of the emotional and technological, Bono calls it a "sci-fi gospel show". It is certainly the most impressive multi-media pop event since, well, the last U2 tour.

Liam and Noel stand at the mixing desk, watching the show wide eyed. Never regarded as the most articulate of people, Liam nonetheless has a distinctive way of expressing himself. "This is the first time I've seen U2," he declares. "Now I understand! It's . . . phwoarghghghgh!" He shakes his head in disbelief, and makes a second attempt at verbalising his enthusiasm. "Fuckin' mad, man. Mad!"

U2 and Oasis have formed something of a mutual admiration society. Backstage after the show, Adam Clayton emerges from a refreshing shower decked out in Manchester City blues. "It's very humble of them and very humbling for us to have them play with us," admits Bono. "They're a great group and its a great moment. And they've been so supportive of us. You get the feeling the Gallaghers could call round into a few houses and sort them out on this U2 soap opera that's going on!"

Liam Gallagher, who appears to have made an early assault on the backstage supplies of alcohol, hijacks the sound system. "You gotta listen to this," he insists. "This is fuckin' great!" It is the new Oasis album, fresh from the studio. The music booms from the speakers, at once recognisable, yet, if anything, fuller, fatter, even more impressive than before. Liam clutches Bono by the shoulders, singing the lyrics of every song directly into his face. Bono - immediately catching hold of a succession of instantly memorable choruses - sings along. The Edge nods his head approvingly. "People say Oasis songs are obvious, but the way the melodies relate to the chords is quite unusual," he observes. "You get the feeling you have heard the songs before, but they still surprise you."

"There's a genius in pulling the obvious from the air," observes Bono, as Liam lurches around the room, lost in the moment, singing along with a new Oasis track, 'Stand By Me'. Only Noel Gallagher would have the gall to write a song with the same title as one of the world's best loved classics. And, right now, only Oasis could carry it off, and make it sound even more classic than the original. "There's a great joy there . . . the joy of pop music is the momentum of the music as it changes, as it morphs into different styles and its success as it carries these groups and these people, surfing, just flying along, that's what pop is," says Bono. Liam, the pop star of the moment, strikingly handsome, moving with the animal grace of some magnificent simian creature, spreads his arms wide and sings, "Stand by me, wherever you a-a-a-are . . ."

A small crowd of backstage revellers - including Winona Ryder and her new beau from Green Day, singers Lisa M and Lisa B, producers Howie B (no relation!), Nelle Hooper and Hal Wilner - watch in open admiration, applauding this astonishing private performance. Liam tunes them all out, eager only to lock Bono into this private world of music. Noel sits on a sofa and takes it all in, a perpetual, secret smile playing on his lips.

"There's a joy in success," Bono insists. "It's a joy you find in club culture, in black music. From Soul II Soul putting out their t-shirts, getting their own culture going, Beastie Boys, Wu-Tang Clan . . . Oasis. Noel Gallagher is taking care of business. It doesn't take away from the soul of what we do. In fact, it helps because there's a sort of fabric of lies around white alternative music. This is what people won't own up to: does everyone want to be in a great group and take it as far as they can? Yes! And anyone who tells you otherwise, they're not telling you the truth. People will take it as far as they can. Some people don't have whatever it needs to go to this level. Well, that's fine. But don't spank us 'cause we can! Because we're obviously not bending over. And no one could accuse Oasis of bending over. We're independent spirits, you know?"

Bono is clearly baffled - perhaps even a little bit hurt - by the negative press surrounding the PopMart tour. "It's not like we're a crap group who are going through the motions and then running back to our fish farms," he declares. "We're doing our best work now. Everyone kind of recognises that. If there is a criticism you could level at us it is for over-reaching. The Rolling Stones go out with a big snake over their head, no one's asking them what that means. We go out with a giant olive and we've got to explain the concept behind it!"

The olive. There it is. That might be the root of the critical problems with the PopMart tour. No matter how hard Bono tries, he knows he is never going to be able to explain the giant olive that perches on top of a giant stick, towering over the stage while U2 play songs of faith and doubt, of passion and reflection, of love and war. Three chords, the truth . . . and an olive on top? Perhaps it is the suggestion of frivolity that critics find hard to comprehend. How can U2 - a group who grapple with the big issues, writing songs that struggle to find meaning in the chaos of life at the fag end of the 20th Century - carry with them as a symbol of their new direction something as meaningless as a giant inflatable olive.

"We're trying to be honest about the size of the group, the scale of the event and the fact that it is a commercial enterprise," says Bono, taking a deep breath and - against his better instincts - trying to explain the thinking behind the PopMart concept. "And we're drawing on, or piggy backing, an entire philosophy that came with the word pop. We're in a commercial world and there's artists like Warhol, and new pop artists like Keith Haring, people who wanted to be part of the real world, they didn't want to be on a gallery wall, they wanted to make prints and make it accessible. Warhol was one of those people who, rather than trying to dodge the contradictions of his situation as an artist working in the commercial world, he actually enjoyed it, mined it, drew from it. He embraced the contradictions.

"There's freedom in that. Freedom for us to get away with these songs, which are spiky, and bitter and there's a brokenness to them. You just wouldn't get away with them unless you surrounded yourself with neon and cosmic glitter. This is the '90s. It's a decade that, in my experience so far, is like this great party and its hangover. And that's what we've put on the record. It starts out like YEAH! and its left you the next morning with a blinding headache and moments of clarity. And I think there's something in facing that, in facing the other side of the party. Because no one can live that life without it turning in on you and getting shallow."

So let me see if I've got this right. The olive represents freedom. And it also symbolises a decade of excess. The night before the millennium after. Well, blame it on the late hour, blame it on the champagne, beer and wine flowing backstage, but this seems to make sense. Sort of. But what about the 40-foot lemon beneath the giant olive? "You've got to have a lemon," says Bono. "A vodka and tonic without the lemon is just not the same thing."

Rounding up the stragglers backstage, Bono announces a visit to Toscas, which is being kept open for the band. It is a favourite venue of his, a bohemian writers enclave, once a haunt of Charles Bukowski, Sam Shepard and Tom Waits (Bono's brother named his Dublin restaurant in honour of Toscas and their father's life long love for opera). The last few diehards pile into a minibus. Noel, pressed next to Bono, clutches the singer's knee as he enthuses about U2 songs he admires. A diligent student of other bands, he displays an impressive knowledge of subtle lyrical twists and musical phrases. And then, with startling synchronicity, the minibus radio, tuned to a late-night station, begins to play U2's hit, 'One'.

"This is the greatest song ever written!" yells Noel. And he and Liam begin to sing it at the top of their voices. Bono, swept away by their exuberance, joins in. And as we roll down a San Francisco highway, long after midnight, three of the world's greatest rock stars treat us to an impassioned, impromptu rendition of a song of unity and brotherly love. "We are one," they sing, "but we're not the same, we've got to carry each other, carry each other . . ."

As the track comes to an end, Bono laughs and hugs Noel's shoulders. "Bands won't admit they like you, right, and you're the greatest band in the world," declares Noel. "And the only band that will actually come out and admit that, is the next greatest band in the world!" n