astronomical unit

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A unit of length used in measuring astronomical distances within the solar system or other planetary system equal to the mean distance from Earth to the sun, approximately 150 million kilometers (93 million miles).

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun astronomy The mean distance from the Earth to the Sun (the semi-major axis of Earth's orbit), approximately 149,600,000 kilometres (symbol AU), used to measure distances in the solar system.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun 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)