Theory, n.; pl. Theories.
Etymology: [F. théorie, L. theoria, Gr. Theater.]
1. A doctrine, or scheme of things, which terminates in speculation or contemplation, without a view to practice; hypothesis; speculation.
Note: "This word is employed by English writers in a very loose and improper sense. It is with them usually convertible into hypothesis, and hypothesis is commonly used as another term for conjecture. The terms theory and theoretical are properly used in opposition to the terms practice and practical. In this sense, they were exclusively employed by the ancients; and in this sense, they are almost exclusively employed by the Continental philosophers." Sir W. Hamilton.
2. An exposition of the general or abstract principles of any science; as, the theory of music.
3. The science, as distinguished from the art; as, the theory and practice of medicine.
2 more senses
4. The philosophical explanation of phenomena, either physical or moral; as, Lavoisier's theory of combustion; Adam Smith's theory of moral sentiments. Atomic theory, Binary theory, etc. See under Atomic, Binary, etc.
Syn. -- Hypothesis, speculation. -- Theory, Hypothesis. A theory is a scheme of the relations subsisting between the parts of a systematic whole; an hypothesis is a tentative conjecture respecting a cause of phenomena.
Shades of meaning
Fernald has no entry of its own for theory; he lists it under hypothesis, idea.
Where he writes about it:
Theory is used of the mental coordination of facts and principles, that may or may not prove correct; a machine may be perfect in theory, but useless in fact. — from hypothesis
Scheme may be used as nearly equivalent to theory, but is more frequently applied to proposed action, and in the sense of a somewhat visionary plan. — from hypothesis