Simply stated: The mean distance between the Earth and the Sun.

For those of you keeping score, that's about 1.49 * 10^13 cm.

The Astronomical Unit (AU) is what is known as a "Natural" Unit. It is a relic of the ancient days of astronomy when no one had any absolute measurements to any other object in the solar system. In these times everything was measured with relative units (i.e. Mars is 1.5 times farther from the Sun than Earth) and later, when absolute measurements were deciphered, no one wanted to give up this unit.

This unit is useful to get a sense of scale for distances that are tough to comprehend. I mean can you really imagine 1.49 *10^13 cm? I thought not. Neither can astronomers.