When is a room not a room?

More and more of London's gallery space is devoted to installations. London is no longer a city, but a vast art puzzle. Next to Creed's FLASHing room is Mike Nelson's installation consisting of an illusionistic labyrinth that seems to lead to a dusty Tate storeroom. It's the security guards I feel sorry for, stuck in a faux back room fielding tricky questions about the aesthetic merits of conceptual art simulacra and helping people with low blood sugar find the way out.

Every London postcode has its installation artist. In SW6 Luca Vitoni has created a small wooden box with grass on the ceiling and blue sky on the floor. Visitors can enhance the experience with free yoga sessions. In W2 the Serpentine Gallery has commissioned Doug Aitken to redesign its space as a sequence of dark, carpeted rooms with dramatic filmed images of icy landscapes, waterfalls and bored subway passengers miraculously swinging like gymnasts around a cross-like arrangement of four video screens. The gallery used to be stables, you know. Not to be outdone, in SE1 Tate Modern has a wonderful installation by Juan Munoz.

At the launch of this year's Turner prize show, a disgruntled painter suggested that the ice cream van that parks outside the Tate should have been shortlisted. This is a particularly stupid idea. Where would we get our ice creams from then?

What we need is the answer to three simple questions. What is installation art? Why has it become so ubiquitous? And why is it so bloody irritating?

First question first. What are installations? "Installations," answers the Thames and Hudson Dictionary of Art and Artists with misplaced self-confidence, "only exist as long as they are installed." Thanks for that. This presumably means that if the ice cream van man took the handbrake off his installation Van No1, it wouldn't be an installation any more.

The dictionary continues more promisingly: installations are "multi-media, multi-dimensional and multi-form works which are created temporarily for a particular space or site either outdoors or indoors, in a museum or gallery."

As a first stab at a definition, this isn't bad. It rules out paintings, sculptures, frescoes and other intuitively non-installational artworks. It also says that anything can be an installation so long as it has art status conferred on it (your FLASHing bulb is not art because it hasn't got the nod from the gallery, so don't bother writing a "funny" letter to the paper suggesting it is). The important question is not "what is art?" but "when is art?"

The only problem is that this definition also leaves out some very good installations. Consider Richard Wilson's 20:50. It consists of a lake of sump oil that uncannily reflects the ceiling of the gallery. Spectators penetrate this lake by walking along an enclosed jetty whose waist-high walls hold the oil at bay. This 1987 work was originally set up in Matt's Gallery in east London, through whose windows one could see a bleak post-industrial landscape while standing on the jetty. The installation, awash in old engine oil, could thus be taken as a comment on Thatcherite destruction of manufacturing industries. Then something very interesting happened. Thatcher's ad man Charles Saatchi put 20:50 in his windowless gallery in west London, depriving it of its context. But the Thames and Hudson definition does not allow that this 20:50 is an installation because it wasn't created for that space. This is silly: it would be better to say there were two installations - the one at Matt's and the other at the Saatchi Gallery.

Or think about Damien Hirst's In and Out of Love. In this 1991 installation, butterfly cocoons were attached to large white canvases. Heat from radiators below the cocoons encouraged them to hatch and flourish briefly. In a separate room, butterflies were embalmed on brightly coloured canvases, their wings weighed down by paint. The spectator needed to move around to appreciate the full impact of the work. Unlike looking at paintings or sculptures, you often need to move through or around installations.

What these two examples suggest to me is that we are barking up the wrong tree by trying to define installations. Installations do not all share a set of essential characteristics. Some will demand audience participation, some will be site-specific, some conceptual gags involving only a light bulb.

Installations, then, are a big, confusing family. Which brings us to the second question. Why are there so many of them around at the moment? There have been installations since Marcel Duchamp put a urinal in a New York gallery in 1917 and called it art. This was the most resonant gesture in 20th century art, discrediting notions of taste, skill and craftsmanship, and suggesting that everyone could be an artist. Futurists, Dadaists and surrealists all made installations. In the 1960s, conceptualists, minimalists and quite possibly maximalists did too. Why so many installations now? After all, two of this year's four Turner prize candidates are installation artists.

American critic Hal Foster thinks he knows why installations are everywhere in modern art. He reckons that the key transformation in Western art since the 1960s has been a shift from what he calls a "vertical" conception to a "horizontal" one. Before then, painters were interested in painting, exploring their medium to its limits. They were vertical. Artists are now less interested in pushing a form as far as it will go, and more in using their work as a terrain on which to evoke feelings or provoke reactions.

"Many artists and critics treat conditions like desire or disease as sites for art," writes Foster. True, photography, painting or sculpture can do the same, but installations have proved most fruitful - perhaps because with installations the formalist weight of the past doesn't bear down so heavily and the artist can more easily explore what concerns them.

Why are installations so bloody irritating, then? Perhaps because in the many cases when craftsmanship is removed, art seems like the emperor's new clothes. Perhaps also because artists are frequently so bound up with the intellectual ramifications of the history of art and the cataclysm of isms, that those who are not steeped in them don't care or understand. But, ultimately, because being irritating need not be a bad thing for a work of art since at least it compels engagement from the viewer.

But irritation isn't the whole story. I don't necessarily understand or like all installation art, but I was moved by Double Bind, Juan Munoz's huge work at Tate Modern. A false mezzanine floor in the turbine hall is full of holes, some real, some trompe l'oeil and a pair of lifts chillingly lit and going up and down, heading nowhere. To get the full impact, and to go beyond mere illusionism, you need to go downstairs and look up through the holes. There are GREy men living in rooms between the floorboards, installations within this installation. It's creepy and beautiful and strange, but you need to make an effort to get something out of it.

The same is true for Martin Creed's Lights Going On and Off, though I didn't find it very illuminating. "My work," says Martin Creed, "is about 50% what I make of it and 50% what people make of it. Meanings are made in people's heads - I can't control them."

It's nice of Creed to share the burden of significance. But sadly for him, few of the spectators were making much of his show last week. His room was often deserted, but the rooms housing Isaac Julien's boring films and Richard Billingham's dull videos were packed. Maybe Creed's aim is to drive people away from installation art, or maybe he is just not understood. Whatever. The lights were on, and sometimes off, but nobody was home.