Acropolis (Greek: Ακρόπολις) means "high city" in Greek, literally city on the extremity and is usually translated into English as Citadel (akros, akron, edge, extremity + polis, city, pl. acropoleis). For purposes of defense, early people naturally chose elevated ground to build a new settlement, frequently a hill with precipitous sides. In many parts of the world, these early citadels became the nuclei of large cities, which grew up on the surrounding lower ground, such as modern Rome.

The word acropolis, although Greek in origin and associated primarily with the Greek cities Athens, Argos, Thebes, and Corinth (with its Acrocorinth), may be applied generically to all such citadels, including Rome, Jerusalem, Celtic Bratislava, many in Asia Minor, or even Castle Rock in Edinburgh. An example in Ireland is the Rock of Cashel.

The most famous example is the Acropolis of Athens, which, by reason of its historical associations and the several famous buildings erected upon it (most notably the Parthenon), is known without qualification as the Acropolis. Although originating in the mainland of Greece, use of the acropolis model quickly spread to Greek colonies such as the Dorian Lato on Crete during the Archaic Period.