Astronomical Unit: Mean distance between Earth and the Sun, common length reference in astronomy

It is the average distance the Earth gets from the Sun on the long axis of the ellipse. Its definition is: the length of the semi-major axis of the Earth's elliptical orbit around the Sun. "Semi-major" means half the long axis.

Astronomical unit
The average distance between the Sun and the Earth is 1 AU
General information
Unit system	Astronomical system of units (Accepted for use with the SI)
Unit of	length
Symbol	au, ua, or AU
Conversions
1 au, ua, or AU in ...	... is equal to ...
metric (SI) units	149597870700 m
imperial & US units	9.2956×107 mi
astronomical units	4.8481×10−6 pc    1.5813×10−5 ly

The AU is about 150 million kilometers or 93 million miles. Astronomers usually measure distances within the Solar System in astronomical units. Mars is about 1.4 AU from the Sun, Jupiter lies at roughly 5.2 AU, and Neptune is roughly 30 AU from the Sun. Light travels an AU in about 8.317 minutes.

From 1976 to 2012 the AU was defined as “the radius of an unperturbed circular Newtonian orbit about the Sun of a particle having infinitesimal mass, moving with a mean motion of 0.01720209895 radians per day (known as the Gaussian constant)”. In 2012, the IAU redefined it to be simply 149597870700 m.

In the IERS numerical standards, the speed of light in a vacuum is defined as c₀ = 299792458 m/s, in accordance with the SI units. The time to cover an AU is τ_A = 499.0047838061±0.00000001 s, resulting in the astronomical unit in metres as c₀τ_A = 149597870700±3 m. It is very roughly the distance from the Earth to the Sun.