Distances in the Solar System are very large! To compare the average distances between the Sun and the planets, it’s convenient to do it in terms of the average Earth-Sun separation.
A very useful approximate definition of the astronomical unit (AU) is:
1 AU = average distance between Sun and Earth = 1.496 × 108km
More accurately, the IAU has defined the AU as:
“equal to the distance from the centre of the Sun at which a particle of negligible mass, in an unperturbed circular orbit, would have an orbital period of 365.2568983 days.”
This is slightly less than the mean Sun-Earth distance.
See also: parsec.