Aristotle, the Greek philosopher, laid it down as a rule that every tragedy, properly constructed, should contain but one catastrophe; should be limited to one denoument; and be circumscribed to the action of one single day. These are called the Aristotelic or Dramatic unities. To these the French have added a fourth, the unity of uniformity, i.e. in tragedy all the “dramatis personæ” should be tragic in style, in comedy comic, and in farce farcical.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894