A set of statements or principles devised to explain a group of facts or phenomena, especially one that has been repeatedly tested or is widely accepted and can be used to make predictions about natural phenomena. The branch of a science or art consisting of its explanatory statements, accepted principles, and methods of analysis, as opposed to practice: a fine musician who had never studied theory. A set of theorems that constitute a systematic view of a branch of mathematics. Abstract reasoning; speculation: a decision based on experience rather than theory. A belief or principle that guides action or assists comprehension or judgment: staked out the house on the theory that criminals usually return to the scene of the crime. An assumption based on limited information or knowledge; a conjecture.