1 Greek Mythology A priestess of Hera who was loved by Zeus. Trying to protect her from the jealousy of Hera, Zeus turned Io into a heifer. Hera sent a gadfly to torture the heifer, which then fled across the world and finally reached Egypt, where Zeus turned her back into human form.
2 Astronomy One of the Galilean moons of Jupiter, the fifth closest satellite to the planet. It is actively volcanic, colored red and yellow with sulfur compounds, and has a diameter of 2,526 miles (3,630 km).