To get pedantic about it, the word "occultism" comes (via the adjective "occult") from the Renaissance Latin phrase philosophia occulta, the hidden philosophy or the philosophy of the hidden forces or factors in the cosmos. "Esotericism" comes from the Hellenistic Greek esoterike, "belonging to the inner" -- usually to an inner circle of people, as distinct from outsiders. So they really do mean pretty much the same thing.