Io

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Greek Mythology A maiden who was loved by Zeus and transformed by Hera into a heifer.
• noun One of the four brightest satellites of Jupiter. It was first sighted by Galileo.

from the GNU version of the Collaborative International Dictionary of English.

• noun An exclamation of joy or triumph; -- often interjectional.
• proper noun In Greek mythology, the beautiful daughter of Inachus, king of Argos, Greece, who was changed by Hera (Juno), in a fit of jealousy, into a white heifer, and placed under the watch of Argus of the hundred eyes. When Argus was killed by Hermes at the command of Zeus, the heifer was maddened by a terrible gadfly sent by Hera, and wandered about until she arrived in Egypt. There she recovered her original shape, and bore Epaphus to Zeus. Epaphus became the ancestor of Ægyptus, Damaus, Cepheus, and Phineus. She was identified by the Egyptians with Isis. According to another legend, Io was carried off by Phoenician traders who landed in Argos. The myth is generally explained to be Aah or the moon wandering in the starry skies, symbolized by the hundred-eyed Argus; her transformation into a horned heifer representing the crescent moon.
• proper noun One of the large moons of the planet Jupiter, remarkable for its intense volcanic activity, as observed in fly-bys of space probes. It was named after the mythological Io.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• proper noun mythology The daughter of Inachus.
• proper noun astronomy A moon of Jupiter, known for its volcanic activity, peppered with about 400 active volcanoes.