theory

n. coherent group of general assumptions, body of principles belonging to a certain subject; speculation, hypothesis

The word theory has a number of distinct meanings in different fields of knowledge, depending on their methodologies and the context of discussion.In common usage, people often use the word theory to signify a conjecture, an opinion, or a speculation. In this usage, a theory is not necessarily based on facts; in other words, it is not required to be consistent with true descriptions of reality. True descriptions of reality are more reflectively understood as statements which would be true independently of what people think about them. In this usage, the word is synonymous with hypothesis. This common usage of theory leads to the common but misguided statement "It's not a fact, it's only a theory."
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Noun
1. 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; "theories can incorporate facts and laws and tested hypotheses"; "true in fact and theory"
(hypernym) explanation
(hyponym) reductionism
(part-meronym) law, law of nature
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; "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
(hypernym) concept, conception, construct
(hyponym) model, theoretical account, framework
(derivation) speculate, theorize, theorise, conjecture, hypothesize, hypothesise, hypothecate, suppose
3. a 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"
(hypernym) belief
(hyponym) patchwork, hodgepodge, jumble
(derivation) speculate, theorize, theorise, conjecture, hypothesize, hypothesise, hypothecate, suppose

(n.)
The science, as distinguished from the art; as, the theory and practice of medicine.
  

(n.)
The philosophical explanation of phenomena, either physical or moral; as, Lavoisier's theory of combustion; Adam Smith's theory of moral sentiments.
  

(n.)
An exposition of the general or abstract principles of any science; as, the theory of music.
  

(n.)
A doctrine, or scheme of things, which terminates in speculation or contemplation, without a view to practice; hypothesis; speculation.
  

In the scientific domain, a well substantiated, comprehensive, predictive explanation, generally approved in the scientific community, but not final, of a phenomenon or set of data.