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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(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.

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The consensus, idea, plan, story, or set of rules that is currently being used to inform a behaviour. This usage is a generalisation and (deliberate) abuse of the technical meaning. "What's the theory on fixing this TECO loss?" "What's the theory on dinner tonight?" ("Chinatown, I guess.") "What's the current theory on letting lusers on during the day?" "The theory behind this change is to fix the following well-known screw...."
(1994-12-14)