Most likely Zone. The acts would play as part of each level (ie, you say green hill zone is a level, and it has 3 acts, rather than referring to each act individually [ie on green hill act 1, 2 and 3, this happens etc etc] )
This might be a dumb question, and it sort of goes off topic, but... If we were talking about Sonic CD, would each level be an act? Because the time differences?
