Three unities

The classical unities, Aristotelian unities, or three unities are rules for drama derived from a passage in Aristotle's Poetics. In their neoclassical form they are as follows:

Aristotle dealt with the unity of action in some detail, under the general subject of "definition of tragedy", where he wrote:

Now, according to our definition, Tragedy is an imitation of an action that is complete, and whole, and of a certain magnitude … As therefore, in the other imitative arts, the imitation is one when the object imitated is one, so the plot, being an imitation of an action, must imitate one action and that a whole, the structural union of the parts being such that, if any one of them is displaced or removed, the whole will be disjointed and disturbed. For a thing whose presence or absence makes no visible difference, is not an organic part of the whole.

His only reference to the time in the fictive world is in a distinction between the epic and tragic forms:

Epic poetry agrees with Tragedy in so far as it is an imitation in verse of characters of a higher type. They differ, in that Epic poetry admits but one kind of metre, and is narrative in form. They differ, again, in their length: for Tragedy endeavours, as far as possible, to confine itself to a single revolution of the sun, or but slightly to exceed this limit; whereas the Epic action has no limits of time.

Unlike his prescriptive attitude regarding the plot (unity of action), Aristotle here merely remarks on the typical duration of a tragedy's action, and does not suggest any kind of imperative that it always ought to be so. He was writing after the golden age of Greek drama, and many Greek playwrights wrote plays that do not fit within these conventions.

Aristotle does not actually mention the neoclassical unity of place at all. However, the prevalent interpretation of his Poetics during the Middle Ages inclined toward interpreting his comment on time as another "unity".

Italian critics of the 16th century, from Lodovico Castelvetro onwards, and then 17th-century French critics, proponents of the neoclassical movement, both expanded Aristotle's descriptions. The result was to make them into prescriptions for structuring plays. French drama of the 17th century, particularly that of Molière and Racine was highly regular, while Corneille incurred harsh criticism for departing from them in Le Cid; whereas the English dramatists writing for the Elizabethan and Jacobean stage were largely unaware of these structures.

...
Wikipedia