Glossary of Literary Theory by Greig E. Henderson and Christopher Brown

Unities:

Action, place, and time, the latter two added to Aristotle's unity of action by Italian and French critics of the sixteenth and seventeenth centuries. Aristotle described tragedy as "an imitation of an action that is serious, complete, and of a certain magnitude" and argued that drama must therefore exhibit "unity of action": an identifiable beginning, middle, and end; a harmonious correlation of whole and parts; a series of events which follow one another inevitably and are related in a causal sequence. Italian and French critics added unity of place -- the dramatic action must be confined to a small geographical area -- and unity of time-it must take place within the confines of a single day.