os'tra siz'm, (from the Greek word meaning shell), a political measure practiced among the ancient Athenians, by which persons considered dangerous to the state were banished by public vote for a term of years (generally ten), with leave to return to the enjoyment of their estates at the end of the period. It takes this name from the shell or tablet on which each person recorded his vote. Among the distinguished persons ostracized were Themistocles, Aristides and Cimon, son of Miltiades. They were afterward recalled.