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Synonyms for Hypothesis (same or very similar meaning)

WordNet sense 1 (a message expressing an opinion based on incomplete evidence):
conjecture, guess, speculation, supposition, surmise, surmisal

WordNet sense 2 (a tentative theory about the natural world; a concept that is not yet verified but that if true would explain certain facts or phenomena):
possibility, theory


From the ODE community, based on WordNetadd/edit