The same thing as artillery, except it's deployed in space, usually from low-Earth orbit, hence the name. The Ion Cannon in Command & Conquer is a good example.
I've heard of ortillery systems being proposed by Star Wars and other SDI programs, in particular the Polyus, developed by the Soviets. I first heard the term used in the Dirtside 2 and Full Thrust manuals (the former is a planetary miniatures game, the latter a space one). | 
