Rule by the mob; in Polybius's account, the degenerate form into which democracy decays when violence and demagogues replace law.
In more detail
Polybius, in Book 6 of the Histories, described a cycle of constitutions from kingship through tyranny, aristocracy, oligarchy and democracy to ochlocracy, and praised the Roman mixed constitution as a check on that decay. The term is now used mainly in historical or polemical contexts, often to dismiss popular rule.