The abbreviation Au or AU may refer to:

Computing

• the .au country code top-level domain for Australia.
• the filename extension for the Sun audio au file format (as in c:\mysong.au)
• Administrative Unit in SDH Networks

Science

• Astronomical Unit (AU)
• Atomic units (au)
• the chemical symbol for gold (Au), from the Latin aurum

Organisations

• African Union
• American University
• Athabasca University

Places

Au is an old Germanic word for rivulet, cognate of severl rivers Aa and with Å in Swedish. It is also the name of several towns and places:

• Germany:
  • Au in der Hallertau
  • Au-Breisgau
• Switzerland:
  • Au, Zürich
  • Au, Thurgau
  • Au, Schaffhausen
• Austria:
  • Au, Austria
  • Au am Anzbach
  • Au am Kraking
  • Au am Leithaberge
  • Au an der Donau
  • Au an der Traun
  • Au bei Brandstatt
  • Au bei der Traun
  • Au bei Ed
  • Au bei Hischmannsberg
  • Au bei hohen Steg
  • Au bei Natternbach
  • Au bei Sirfling
  • Au bei Turnau

Other

• ISO 3166-1 code for Australia; see ISO 3166-2:AU for subdivisions.
• a brand of Japanese mobile phone operator KDDI
• Alternative Universe (AU), a fan fiction term
• a 'cartwheel' in the Brazilian martial art of Capoeira
• Au (區 or 歐) is a family name in Hong Kong