My game has a 2-tier hierarchy of levels, it is composed of 6 or more "outer levels" that consist of 8 or more "inner levels". Reaching an inner level unlocks the next inner level. Reaching the top inner level of an outer level unlocks the next outer level. The first inner levels of an outer level resemble the inner levels of the previous outer level, but each outer level has more inner levels than its predecessor.
What terms are common to refer to "outer levels" and "inner levels" in such a situation? The game is very abstract, so something like "world" for the outer level would be inappropriate.