1. a proposal intended to explain certain facts or observations
2. a tentative insight into the natural world; a concept that is not yet verified but that if true would explain certain facts or phenomena
a scientific hypothesis that survives experimental testing becomes a scientific theory|he proposed a fresh theory of alkalis that later was accepted in chemical practices
3. a message expressing an opinion based on incomplete evidence
4. A supposition; a proposition or principle which is supposed or taken for granted, in order to draw a conclusion or inference for proof of the point in question; something not proved, but assumed for the purpose of argument, or to account for a fact or an occurrence; as, the hypothesis that head winds detain an overdue steamer.