Infinite baffle is a pretty self-descriptive term. If you use Gregm's definition (which is correct), just think of the front baffle of any speaker in which all drivers are front mounted. Now imagine that that baffle is infinitely large. There willl be no front/back wave interaction because the waves are separated by an "infinite" baffle. In the real world the baffle is merely large (not infinitely large), so a large sealed box is required to isolate the rear waves.
Marty
Marty