IONIUM MARE, a term in use among the ancient geographers for that part of the Mediterranean which washed the western shores of Greece and Epirus, and separated them from Italy and Sicily. The origin of the name, and its exact purport as at first employed, are alike very doubtful. The term is first found in Æschylus, though what meaning he attached to the words cannot be clearly ascertained. Herodotus and Thucydides both talk of the "Ionian Gulf," which, as described by them, can only mean the Adriatic; and the first of these historians often alludes to the "Ionian Strait," indicating by that term the passage between the heel of Italy and the Epirote coast. In course of time a wider but vague and indefinite meaning was attached to the term. Polybius employed it to indicate a part of the Mediterranean extending from the entrance of the Adriatic southwards to the Peloponnese. At a later period Pliny included under it the Sicilian and Cretan seas. Later still all these arbitrary subdivisions of the Mediterranean were merged in the extension of the name Adriatic, which comprised them all. See ADRIATIC SEA.