“Is that a tattoo of a hop?”

There’s something green poking out of Stuart Ross’ sleeve. Ross smiles and nods. On the back of his hand sits a bright green bud. “I’ve got vines of them going all up my other arm and round my shoulder,” he says proudly.

Someone jokingly asks if Ross knows what variety of hop it is. Ross is not fazed.

“It’s a new experimental one they’re cultivating to try to get American-style hop aromas into plants grown in Kent,” he replies. “I don’t know if it was one of the successful trials, or whether it ended up in the bin.”

“Paul Corbett from Charles Faram hop merchants is at the bar next door,” says one of the other drinkers at the table. “Why don’t you go and ask him?”

Ross nods, gets up and goes to identify his hop.

While tattoos of specialist hop varieties might be an extreme expression of craft beer love, Ross is not alone in his affections. Craft beer is on a roll. This week, the Office of National Statistics announced that, along with e-cigarettes, craft beer has been added to the basket of goods that are used to calculate the rate of inflation. Here in the Sheffield Tap – a stunningly restored pub in Sheffield train station – it’s easy to see why. Men and women of all ages and appearances queue at the bar to choose from more than 20 taps of hoppy ales, wheat beers and imported authentic pilsners, and a range of several hundred bottles of beer from conventional to, well, seaweed ale and “barley champagne”.

The latest figures from market researchers Mintel show that one in five British adults have drunk craft beer in the last six months. The number of breweries in the UK has trebled since the millennium to more than 1,400, despite the total beer market shrinking by around a quarter during that time. London, which had two breweries in 2006, now boasts close to 70. Craft has made beer cool and interesting to new generations of drinkers. It’s the biggest thing to hit beer in living memory.

There’s just one problem: no one involved in this thing seems to be able to agree on what it actually is. And when they argue about the distinctions, things can get a bit heated.

With his tattoos and Rasputin-like beard, Ross would be a cliche of the east London hipster brewer were it not for his broad Yorkshire burr. Craft beer has been dismissed by some as nothing more than a hipster affectation, a fashion accessory to go with beards and tats. But Ross was brewing craft beer, even if he wasn’t calling it that, in Sheffield in 2008, when his number-one crop and neat goatee were conventionally South Yorkshire. Now, as head brewer of Magic Rock – one of the most celebrated craft breweries in the UK – his appearance, like his beer, owes more to the west coast of the US.

Beers on sale at the Sheffield convention. Photograph: Gary Calton

Craft in the US is a straightforward proposition. There, the Brewers’ Association defines a craft brewer as one that is small, independent and traditional, and keeps a tight grip on who gets to be included. But in the UK, there is no comparable definition. Craft accounts for 2%, 10% or maybe 20% of the market, depending on who you ask, and how they define it.

The Society of Independent Brewers (Siba) was founded in 1980, to represent the interests of what were then called microbrewers. Today, Siba has 800 brewery members, and its annual conference in Sheffield dominates what has always been a beery city full of small breweries. As the biggest gathering of craft brewers in the UK, it seems the perfect place to nail and articulate what craft beer is. But new Siba CEO Mike Benner is careful not to use the word “craft” as he addresses the conference on the huge growth and dynamism in small-scale, independent British brewing. The word doesn’t appear at all in the organisation’s new strategic plan.

“We’ve debated it for years, and our membership is split down the middle as to whether we should call ourselves craft brewers or not,” he tells me after his speech.

Rupert Thompson, who runs Surrey’s Hog’s Back brewery, chips in: “I’d probably describe us as craft, but with some reticence. We used to describe our lager as ‘English craft lager’, and then I took the word ‘craft’ off again, because I didn’t want it to look like we were bandwagon jumping. I’m still not sure we were right to take it off.”

The British have always been less comfortable accepting labels than the Americans but there’s much more to Benner’s reticence and Thompson’s unease around the term than that. When craft beer emerged in the US, it stood against a market dominated by three identical-tasting commercial lagers. It clearly stood for quality and flavour. But here in the UK, there was already a product that tasted very different from American craft beer, but ticked all the boxes of being traditional, small scale and independent: real ale. So is craft beer the same thing as real ale? This is where the headaches begin, before you’ve even touched a drop.

When the UK’s Campaign for Real Ale (Camra) was founded in 1971, it identified the difference between good and bad beer was that good beer was fresh, live, unpasteurised and served from a traditional cask, while bad beer was filtered, pasteurised, artificially fizzy (thanks to the addition of carbon dioxide) and served from pressurised kegs.

Many in Camra still believe that cask is good and keg is bad. American craft beer shares some characteristics with cask ale, but it’s often filtered and served from kegs, bottles and even cans. In 2011, Camra chairman Colin Valentine launched a scathing attack on craft beer and the “bloggerati” who championed it, warning that craft “is served using CO2 and/or nitrogen. It is keg beer. It may have hops in it, but it is keg.”

When I put this to Bob Pease, CEO of the American Brewer’s Association and key speaker at the Siba conference, he visibly bristles at the implication that keg beer is somehow inferior to cask. “That would be an affront to American craft brewers. We may not get cask ale, but we have a total appreciation of quality, in any format.”

The beer bloggers appear to agree with Pease, often painting Camra as out of touch, and attacking British real ale as “boring brown beer”. But Pease doesn’t welcome support in this form.

A lot of bottle ... craft beer on sale. Photograph: Gary Calton

“We wouldn’t agree with that at all. American craft brewers were inspired by British tradition and flavour. We just put our own spin on it. It’s great that American-style beers are doing so well here, but real ale is obviously craft beer. It’s a circular effect.”

I phone Colin Valentine for an opposing view, expecting the same anger as in his conference speech a few years ago. If I’m looking for fireworks, however, I’m disappointed.

“Craft beer is a nebulous concept, it means what you want it to mean,” he says. “Camra is the campaign for cask ale, which we believe is the zenith of the brewer’s art. But if craft beer gets people interested in better beer generally, it’s good for all of us.”

Siba’s Mike Benner agrees. “There’s no need for craft to be defined any further. Drinkers define their own terms, and people choosing craft have already decided what it is.”

Mintel’s research shows that a third of craft drinkers don’t really know what it is, but it doesn’t seem to matter. While debates still rage online (especially in the comments on Guardian articles about craft beer) it seems the brewing industry has reached an accord that craft is hazily defined, but useful.

Apart from in one aspect.

Craft beer is becoming big business, which means the big brewers – those that craft defines itself against – want a piece of the action. In the US, at the same time as running attack ads on craft beer and the people who drink it, Bud owner Anheuser-Busch Inbev is snapping up breweries that make the very craft products it publicly disdains.

Over here, big traditional ale brewers are launching their own “craft” ranges. Some of their beers are OK. But if the “enemies” of craft beer start saying they’re craft themselves, does craft become a debased, meaningless term?

“The difference between us and the big guys is that they’re not beer people. I don’t believe they even drink beer,” says Richard Burhouse, founder of Magic Rock. “They might be able to brew decent beer, but they’re doing it for the wrong reasons. They’re just trying to protect their market share; it doesn’t come from within. They’re always responding to the market, and that’s why they’re always late. If they really cared about beer, they’d have done it earlier.”

The British brewer that is named more often in this respect than any other (though not specifically by Burhouse) is Greene King. Hugely popular, with an estate of 2,000 tied pubs and some of the country’s biggest ale brands, it’s often attacked for being big and bland. The launch of a Greene King “craft” range in 2013 brought angry howls of derision.

The criticism doesn’t seem to faze Greene King’s marketing director, Dom South. “Greene King brings reliability and stability to a market that can be a bit unreliable,” he says. “We’ve found drinkers aren’t too bothered about the size of the brewery. They judge a beer on what’s in front of them. And anyway, I really wouldn’t class us as one of the big boys. We’re the biggest of the small brewers. We’re tiny compared to the likes of Anheuser-Busch or Molson Coors.”

Back in the Sheffield Tap, Stuart Ross has returned from his investigations, delighted to have learned that the hop variety inked his hand is now in commercial production. I ask him if he agrees with Richard that bigger brewers pose a threat to craft beer.

He shrugs. “They might be jumping on the bandwagon, but they could end up helping us. If they’re half-way between factory-produced lager and what we do, it might bring in more people and introduce them to more interesting beer.”

I realise that these debates about the meaning, or even the existence of craft beer – debates that must seem amusingly eccentric or boring and nerdish to some – only happen because everyone arguing cares so much about good beer. They might disagree over what (and who) makes a good beer, but they think it’s too important to be left unexamined. For the more mainstream audience, who don’t really care about the relative merits of cask and keg, or the size of the brewery that made their beer, craft is a useful shorthand for quality, flavour and integrity. It’s something a bit different from the normal, mass-produced homogeneity; something worth paying a bit more for. And that’s all the definition it needs.

A very drunk man approaches our table, waving his pint glass alarmingly above our heads. “I can tell your lot are southern,” he slurs. He informs us that he owns two pubs in the area, but now lives in Spain. Then he gestures at the shining inhouse brewery behind us, on which the Sheffield Tap brews several of its own beers. “Look at that,” he says. “You lot won’t know anything about this, coming from London, but that’s beautiful, look at it. It’s the future, is that. Beautiful.” And off he goes.