Environmental is an uncommon arcade game form factor.
The environmental form factor basically begins and ends with Tron and Discs of Tron (there were a few others, and some Tron machines were later converted, but these are the only ones worth mentioning).
In an environmental machine the player is completely enclosed inside the game, but they play the game standing up (as opposed to cockpit machines where the players sit down). Most environmental machines are fully tricked out with black lights, mirrors, multiple audio channels, and other gimmicks.
Environmental machines are a favorite with collectors due to their complexity, good looks, and the sheer quality of the few games available in environmental cabinets. The prices for these games on the used market are almost always in the four figure range and are rising all the time. When purchasing an environmental cabinet you should always examine it very closely, these are some of the most complex games ever made, and it is very uncommon to find one with everything inside of it in working order. You should also make sure that the machine hasn't been sawed in half, that is a problem with these machines, as they are difficult to transport, and are often separated into two pieces by some forward thinking individual.