An astronomical unit (abbreviated as AU, au, a.u., or ua) is a unit of length equal to 149,597,870.700 kilometres (92,955,807.2730 mi) or approximately the mean Earth–Sun distance.

The symbol ua is recommended by the International Bureau of Weights and Measures, and the international standard ISO 80000, while au is recommended by the International Astronomical Union, and is more common in Anglosphere countries. In general, the International System of Units only uses capital letters for the symbols of units which are named after individual scientists, while au or a.u. can also mean atomic unit or even arbitrary unit. However, the use of AU to refer to the astronomical unit is widespread. The astronomical constant whose value is one astronomical unit is referred to as unit distance and is given the symbol A.

The AU was originally defined as the length of the semi-major axis of the Earth's elliptical orbit around the Sun. In 1976 the International Astronomical Union revised the definition of the AU for greater precision, defining it as that length for which the Gaussian gravitational constant (k) takes the value 0.017 202 098 95 when the units of measurement are the astronomical units of length, mass and time. An equivalent definition is the radius of an unperturbed circular Newtonian orbit about the Sun of a particle having infinitesimal mass, moving with an angular frequency of 0.017 202 098 95 radians per day; or that length such that, when used to describe the positions of the objects in the Solar System, the heliocentric gravitational constant (the product GM_☉) is equal to (0.017 202 098 95)² AU³/d². The value of the Astronomical Unit must be obtained experimentally and so is not known exactly.