stab at a definition

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.