"Arena" is a heavily overloaded term. Usually arenas have more logic than a mere bump allocator though. Sometimes "arena" means "thread-local cache", so objects can be free'd to a different arena than they came from, etc. Arenas might be a single finite-size underlying allocation or contain multiple of those dynamically. Arenas might be able to answer the question "did this arena allocate this pointer originally"? Arenas might be user-facing objects or internal implementation details only.