Site-specific theatre is any type of theatrical production designed to be performed at a unique, specially adapted location other than a standard theatre. This specific site either may be originally built without any intention of serving theatrical purposes (for example, in a hotel, courtyard, or converted building), or may simply be considered an unconventional theatre space (for example, in a forest).
When the location is meant to imitate, or is itself, the setting of the theatrical story (as is common with site-specific theatre), the performance may also then be called environmental theatre. Site-specific theatre is commonly more interactive than conventional theatre and, with the expectation of audience members predominantly to walk or move about (rather than sit), may be called promenade theatre. Site-specific theatre frequently takes place in structures originally built for non-theatrical reasons that have since been renovated or converted for new, performance-based functions.
Bored Of Studies Article on site specific or environmental theatre.
Greenmuseum.org Quick definition of environmental art, in general.
Helium Useful definition of environmental theatre.