noun pl. theories -·ries

1. Obsolete a mental viewing; contemplation
2. a speculative idea or plan as to how something might be done
3. a systematic statement of principles involved the theory of equations in mathematics
4. a formulation of apparent relationships or underlying principles of certain observed phenomena which has been verified to some degree
5. that branch of an art or science consisting in a knowledge of its principles and methods rather than in its practice; pure, as opposed to applied, science, etc.
6. popularly a mere conjecture, or guess

Etymology: < Fr or LL: Fr théorie < LL theoria < Gr theōria, a looking at, contemplation, speculation, theory < theōrein: see theorem