An Astronomical Unit (AU) is approximately the mean distance between the Earth and the Sun and is equal to 149,597,870.691 kilometers
It is a derived constant and used to indicate distances within the solar system. Its formal definition is the radius of an unperturbed circular orbit a massless body would revolve about the sun in 2*(pi)/k days (i.e., 365.2568983.... days), where k is defined as the Gaussian constant exactly equal to 0.01720209895. Since an AU is based on radius of a circular orbit, one AU is actually slightly less than the average distance between the Earth and the Sun (approximately 150 million km or 93 million miles).
All civilian space ships (see Complete ship object list) gauge inter-sytem distances in this unit.