When is a room not a room?
When is a room not a room? There was a bit of a fuss at Tate Britain the other day. A woman was hurrying through the large room that houses Lights Going On and Off in a Gallery, Martin Creed's Turner prize-shortlisted installation in which, yes, lights go on and off in a gallery. Suddenly the woman's necklace broke and the beads spilled over the floor. As we bent down to pick them up, one man said: "Perhaps this is part of the installation." Another replied: "Surely that would make it performance art rather than an installation." "Or a happening," said a third.
These are confusing times for Britain's growing audience for visual art. Even one of Creed's friends recently contacted a newspaper diarist to say that he had visited three galleries at which Creed's work was on show but had not managed to find the artworks. If he can't find them, what chance have we got?

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