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1590s, "conception, mental scheme," from Late Latin theoria (Jerome), from Greek theoria "contemplation, speculation, a looking at, things looked at," from theorein "to consider, speculate, look at," from theoros "spectator," from thea "a view" + horan "to see" (see warrant (n.)). Sense of "principles or methods of a science or art (rather than its practice)" is first recorded 1610s. That of "an explanation based on observation and reasoning" is from 1630s.
theory the·o·ry (thē'ə-rē, thēr'ē)
A systematically organized body of knowledge applicable in a relatively wide variety of circumstances, especially a system of assumptions, accepted principles, and rules of procedure devised to analyze, predict, or otherwise explain the nature or behavior of a specified set of phenomena.
Abstract reasoning; speculation.
A set of statements or principles devised to explain a group of facts or phenomena. Most theories that are accepted by scientists have been repeatedly tested by experiments and can be used to make predictions about natural phenomena. See Note at hypothesis.
In science, an explanation or model that covers a substantial group of occurrences in nature and has been confirmed by a substantial number of experiments and observations. A theory is more general and better verified than a hypothesis. (See Big Bang theory, evolution, and relativity.)