If someone invites me over to watch a movie on her/his "home theater" the only absolute requirement I have come to expect is some sort of external speaker system attached to a viewing screen be it a tv or projection screen. For me the term home theater is very nebulous because I've heard people with a couple externally powered speakers connected to a tv as well as people with a full blown dedicated movie screen and a jillion speakers all refer to their system as a home theater.

Doesn't bother me either way. Only thing that would bug me is if someone only had a tv and a bag of popcorn then referred to that as a home theater.