A Latin interjection, or exclamation of joy or triumph: sometimes used as a noun in English.n. In mythology, a daughter of Inachus, metamorphosed into a heifer and caused to be tormented by a terrible gadfly by Hera, in jealous revenge for the favors of Zeus. See Argus, 1.n. The innermost of the four satellites of Jupiter.n. In entomology: A genus of vanessoid butterflies.n. [lowercase] The peacock butterfly, Vanessa io: used both as the technical specific name and as an English word.n. [lowercase] A showy and beautiful moth of North America, Hyperchiria io, or Saturnia io, of yellow coloration, with prominent pink and bluish eyes on the hinder wings.n. An abbreviation of Iowa.