Aristotle’s Definition of Tragedy

Tragedy, then, is an imitation of an action that is serious, complete, and of a certain magnitude; in language embellished with each kind of artistic ornament, the several kinds being found in separate parts of the play; in the form of action, not of narrative; through pity and fear effecting the proper purgation of these emotions. By 'language embellished,' I mean language into which rhythm, 'harmony' and song enter. By 'the several kinds in separate parts,' I mean, that some parts are rendered through the medium of verse alone, others again with the aid of song.

- from Poetics