noun (n)

• a 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(noun.cognition)

  Example:
  True in fact and theory.
  

  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.
  He proposed a fresh theory of alkalis that later was accepted in chemical practices.
  

  Synonym:
  hypothesis, possibility
  

  source: wordnet30
• a belief that can guide behavior(noun.cognition)

  Example:
  The architect has a theory that more is less.
  They killed him on the theory that dead men tell no tales.
  

  source: wordnet30
• A doctrine, or scheme of things, which terminates in speculation or contemplation, without a view to practice; hypothesis; speculation.(noun)

  source: webster1913