What does "unless" mean?

Unless: Upon any less condition than (the fact or thing stated in the sentence or clause which follows); if not; supposing that not; if it be not; were it not that; except; as, we shall fail unless we are industrious. Note: By the omission of the verb in the dependent clause, unless was frequently used prepositionally, -- a construction common in Shakespeare and still employed colloquially. Here nothing breeds unless the nightly owl. Shak.