Definition Proper noun : A moon of Jupiter : In astronomy, Io is the innermost of the four large Galilean moons of the planet Jupiter. It is notable for its intense volcanic activity. A figure in Greek mythology : In classical mythology, Io was a mortal princess who was seduced by Zeus (Jupiter). To hide her from his wife Hera, Zeus transformed Io into a white heifer. Usage Examples Proper noun...

See full definition →