noun (n)

• a proposal intended to explain certain facts or observations(noun.communication)

  source: wordnet30
• a tentative insight into the natural world; a concept that is not yet verified but that if true would explain certain facts or phenomena(noun.cognition)

  Example:
  A scientific hypothesis that survives experimental testing becomes a scientific theory.
  

  Synonym:
  possibility, theory
  

  source: wordnet30
• a message expressing an opinion based on incomplete evidence(noun.communication)

  Synonym:
  conjecture, guess, speculation, supposition, surmisal, surmise
  

  source: wordnet30
• 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.(noun)

  source: webster1913