|MadSci Network: Astronomy|
A.U. is an abbreviation for Astronomical Unit, which the American Heritage Dictionary defines as: "A unit of length used in measuring astronomical distances within the solar system equal to the mean distance from Earth to the sun, approximately 150 million kilometers (93 million miles)."
Because the Earth's orbit is eliptical, an A.U. is the average between the shortest and furthest distances of the Earth to the sun. This is a very useful reference for distances within a solar system, since it fills the measurement gap between a kilometer and a lightyear.
So next time someone asks how far the Earth is from the sun, tell them,