From 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]

From WordNet (r) 2.0:

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,
           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]

From Virtual Entity of Relevant Acronyms (Version 1.9, June 2002):

AU
     Access Unit
     
     

From The Free On-line Dictionary of Computing (27 SEP 03):

au
     
        1. <networking> The two character country code for Australia
        used in Internet domain names.
     
        2. <filename extension> audio.
     
        (1995-02-15)