The Collaborative International Dictionary of English v.0.48:

Io \I"o\, n.; pl. Ios. [L.; cf. Gr. "iw`.]
   An exclamation of joy or triumph; -- often interjectional.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Io \I"o\ ([=e]"[=o]), prop. n. [Gr. 'Iw`.]
   1. 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 [AE]gyptus, 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.
      [Century Dict. 1906]

            Greek mythology, too, knew her [Astarte] as I[^o]
            and Europa, and she was fitly symbolised by the cow
            whose horns resemble the supine lunar crescent as
            seen in the south.                    --Seyce (Anc.
                                                  Empires, p.
                                                  195).
      [Century Dict. 1906]

   2. 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.

   Note: The remarkable images of Io and Jupiter obtained by the
         Galileo 2 space probe to Jupiter and Hubble photographs
         are available at the web site of [a
         href="http:]/www.nasa.gov">NASA. Reproduced here are:
         Surface of Io.
         Full disk of Io passing, over the clouds of Jupiter.
         Jupiter with Io in transit.
         More images are available at the NASA site. [PJC]

V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

IO
       Input/Output, "I/O"


The Free On-line Dictionary of Computing (30 December 2018):

I/O

   Input/Output