The term installation was coined in the 1970s to describe artwork that of any form or size that inhibits a space inside or outside a gallery, which is often site-specific. 

Installations are typically three-dimensional and can be made up by a variety of material and medium. They can be indefinite or temporary and viewers can find themselves walking around a piece, contemplating it from a distance or immersing themselves in the space of the work itself. 

It is common for installations to be specially commissioned for sites.  If someone is interested in buying a site-specific installation an artist may be willing to adapt the original for a new setting or create a similar piece.

Norah Stone, Collector, San Francisco. Owning Art: The Contemporary Art Collector’s Handbook by Louisa Buck and Judith Greer, Cultureshock Media, 2006