Original: An astronomical unit (abbreviated as AU, au or a.u.) is a unit of length equal to about 149,597,870.7 kilometres (92,955,807.27 miles) or approximately the mean Earthâ "Sun distance.
Simplified: The astronomical unit (AU or au or a.u. or sometimes ua) is a unit of length about the same as the mean distance from the center of the Earth to the center of the Sun.