Open English Wordnet
Nouns
(n)theorya well-substantiated explanation of some aspect of the natural world; an organized system of accepted knowledge that applies in a variety of circumstances to explain a specific set of phenomena“theories can incorporate facts and laws and tested hypotheses”,
“true in fact and theory” (n)hypothesis, possibility, theorya 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” (n)theorya belief that can guide behavior“the architect has a theory that more is less”,
“they killed him on the theory that dead men tell no tales”