Etymology
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theory (n.)

1590s, "conception, mental scheme," from Late Latin theoria (Jerome), from Greek theōria "contemplation, speculation; a looking at, viewing; a sight, show, spectacle, things looked at," from theōrein "to consider, speculate, look at," from theōros "spectator," from thea "a view" (see theater) + horan "to see," which is possibly from PIE root *wer- (3) "to perceive." Philosophy credits sense evolution in the Greek word to Pythagoras.

Earlier in this sense was theorical (n.), late 15c. Sense of "principles or methods of a science or art" (rather than its practice) is first recorded 1610s (as in music theory, which is the science of musical composition, apart from practice or performance). Sense of "an intelligible explanation based on observation and reasoning" is from 1630s.

updated on April 18, 2022

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Definitions of theory from WordNet

theory (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;
true in fact and theory
theory (n.)
a tentative insight into the natural world; a concept that is not yet verified but that if true would explain certain facts or phenomena;
he proposed a fresh theory of alkalis that later was accepted in chemical practices
a scientific hypothesis that survives experimental testing becomes a scientific theory
Synonyms: hypothesis / possibility
theory (n.)
a belief that can guide behavior;
they killed him on the theory that dead men tell no tales
the architect has a theory that more is less
From wordnet.princeton.edu, not affiliated with etymonline.