At the mouth, southern side, of the Corinthian Gulf in central Greece. (Antirrhion -- "Opposite-Rhion" -- stands on the corresponding northern side; Strabo 8.2.3 (cf. 9.1.1) records the distance between them as 'only about five stades'; and since 2004 they have been joined by the Charilaos Trikoupis, a.k.a. Rio-Antirio, Bridge.)
From the scholia to Homer, Iliad 14.225, where -- as often elsewhere -- the lower-case version of the headword appears; see generally LSJ s.v. (and rho 157).