Lewis & Short

Īōnĭus, a, um, adj., = Ἰώνιος or Ἰόννιος,

  1. I. of or belonging to Ionia, Ionian: attagen, Plin. 10, 48, 68, § 133: Ionium mare (or poet. aequor), or Ionii fluctus, Ionius sinus; or subst.: Ionium, i, n., the Ionic Sea, in the west of Greece: mare, Mel. 1, 3, 3; Plin. 3, 8, 14, § 88; Liv. 23, 33, 22; Verg. A. 5, 193: aequor, Ov. M. 15, 700: fluctus, Verg. G. 2, 108: sinus, Hor. Epod. 10, 19; and simply Ionium: insulae Ionio in magno, Verg. A. 3, 211: per Ionium vectus, Prop. 3 (4), 21, 19 (but Ionia is a false reading for Jovis, Plin. 37, 7, 29, § 103).
  2. II. Subst.: Ĭōnĭa, ae, f., = Ἰωνία, Ionia, a country of Asia Minor on the Ægean Sea, between Caria and Æolis, Mel. 1, 17; 2, 7, 4; Plin. 5, 29, 31, § 112; Nep. Alc. 5, 6; Prop. 1, 6, 31; Ov. F. 6, 175 al.