As has been pointed out, we are talking about two things here:
- art called "installation art" or called an "art installation"
- the act of installing or putting in/up art
The act of installing art doesn't need much explanation. Works of art referred to as "installation pieces" are usually works that do not hang on walls and are not traditional statues, as described by posts 3, 4, and 6.
I recall a few years back that an artist placed digital motion sensory panels beside a road somewhere in Europe. The panels, which were about 15 feet tall, would change colors or even flash a wavy design everytime a car drove by, thus sort of interacting with both the space they were located and with passing vehicles. Perhaps kind of dangerous if they distracted drivers (though no more, I suppose than a billboard), but an example of installation art.
When I think of the term "art installation," I think of that Christo person. His stuff is like what Post 4 is talking about -- it's always something temporary. He's the guy who does stuff like wrapping islands in plastic or putting up a lot of plastic gates in Central Park in NY City. That's my vision of an art installation.
I heard on the news today that a new upscale retail store is being opened in my area. I was surprised to hear that part of the grand opening included highlighting art designed for the occasion by local artists. I had never heard of such a thing.
"Art installation" would usually refer to the process of bringing a work of art into the area in which it is going to be displayed. This might be most frequently used in connection with works of art intended for permanent and/or public display.
For example, before a large work of sculpture can go on display in a public plaza, the area in which it will be located needs to be prepared by insuring that footings are in place to support the weight of the sculpture, creating a platform or base for the sculpture, bringing in the sculpture itself and attaching it firmly to the base, adding any lighting or security equipment that may be required, and arranging the surrounding area so as to complement the sculptured art.
The same basic process would be needed for other types of artwork.
Installation art describes an artistic genre of three-dimensional works that are often site-specific and designed to transform the perception of a space. Generally, the term is applied to interior spaces, whereas exterior interventions are often called Land art; however, the boundaries between these terms overlap.
Wordprof is correct, and I would add that an art installation is more often then not a temporary affair. The artist must take into considerastion the space that the installation will be presented in. Art installations can range from a simple presentation to one as complex as utilizing video, sounds, projections, objects, lights, or any other large number of options available tio the artist. The simpler installations tend to be more successful than the complex ones as the more forms used the better the possiblity of them canceling each other out, which could, or may one day be. exactly the intent of the artist.