definition of
AU

[1] #1 AU n. [gold, Au, atomic number 79] 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. wn

[2] #2 AU n. [Astronomical Unit] 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). wn