(anciently Sphairion), an island of Greece, at the S.E. entrance of the Gulf of Ægina, separated from the mainland by a narrow channel. It is hilly and of volcanic origin. A town of the same name occupies a singular position among the dark-coloured rocks on the south coast. Poros is the national arsenal of Greece, and has a small naval yard and a harbour between the island and the mainland. Pop. 7000, chiefly Albanians.