(1):

(n.) A doctrine, or scheme of things, which terminates in speculation or contemplation, without a view to practice; hypothesis; speculation.

(2):

(n.) An exposition of the general or abstract principles of any science; as, the theory of music.

(3):

(n.) The science, as distinguished from the art; as, the theory and practice of medicine.

(4):

(n.) The philosophical explanation of phenomena, either physical or moral; as, Lavoisier's theory of combustion; Adam Smith's theory of moral sentiments.