Theatre is an outdoor structure for drama performances or spectacles in ancient Greece and Rome.
The oldest place which can be called a theatre is situated in Knossos and we don’t know what kind of spectacle the inhabitants of Knossos watched there; it’s highly likely though that they watched beautiful Cretan girls gracefully whirl around in what Homer calls “the dancing place”. Dance is the origin of theatre and that’s why the very heart of the theatre, its core, is called orchestra: the area where the chorus “orchestrates”.
The concept of theatre is deeply connected with the concept of Greek civilization and if we paraphrased Pausanias, we could say that a city cannot be named a city if it has no theatre. So, Greeks founded dozens of theatres all over the ancient world. Even Spartans had a theatre, although we are accustomed to thinking of them always fighting or exercising.
Naturally Athens holds once more the leading part and the earliest extant theatre today lies on the southern slope of the Acropolis. Despite the fact that what we see today reflects the form the theatre took in the 4th century in the time of the archon Lycurgus, we cannot help thinking that in this theatre presented their plays the great tragic poets Aeschylus, Sophocles and Euripides and in this same theatre once sat among the public Solon, Pericles, Socrates, Plato and Aristotle who gave us the definition of tragedy.
Many and important theatres were built by the Greeks, in Thoricos and Corinth, in Delphi and Delos, Dodona and Megalolpolis, Syracusae and Milos and the most recently excavated in Messini. The most beautiful and best preserved though, lies in Epidaurus.
Source : http://www.visitgreece.gr/en/culture/tourist_guides_take_us_around_ancient_theatres