Forum OpenACS Q&A: Re: What is a Community in 5.0?
Subsite has no meaning in any normal dictionary whatsoever.
I do believe that community tends to have a meaning to most people, and it surely have a meaning in a dictionary. Even ArsDigita once had a tag line along the lines of "purposeful communities".
I'm absolutely willing to subject this to a usability test, but lacking that, my impression is that community makes a hack of a lot more sense that "subsite".