The Collaborative International Dictionary of English v.0.48:

au \a.u.\ n.
   1. (Astron.) the unit of length equal to the mean distance of
      the Earth from the sun, about 93 million miles (150
      million kiometers); -- used almost exclusively in
      astronomy, or to describe astronomical distances.

   Syn: astronomical unit, AU, A.U.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

Au
    n 1: a soft yellow malleable ductile (trivalent and univalent)
         metallic element; occurs mainly as nuggets in rocks and
         alluvial deposits; does not react with most chemicals but
         is attacked by chlorine and aqua regia [syn: gold, Au,
         atomic number 79]
    2: a unit of length used for distances within the solar system;
       equal to the mean distance between the Earth and the Sun
       (approximately 93 million miles or 150 million kilometers)
       [syn: Astronomical Unit, AU]

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

AU
       Access Unit


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

au

   1.  The two character country code for Australia
   used in Internet domain names.

   2.  audio.

   (1995-02-15)