I always thought it was a small "church" in another building, or a non-denominational place of worship.
The vocabulary I have known is thus:
A "church" is the entire building. Meeting rooms, choir rooms, multi-purpose rooms were all part of the church.
The "narthex" is the area just outside the nave (the place of worship).
The "nave" is the area in which the congregation sits and worships (often mis-labelled the sanctuary).
The "sanctuary" or "chancel" is the holiest part of the church. The area surrounding the altar, and it is usually separated from the nave by means of a railing or partition.
I know these terms apply to protestant churches, and I'm fairly certain they also apply to Catholic and Orthodox.
Non-Christian and smaller sects, of course, use different vocabularies.