a.u.

Initialism of arbitrary unit.

Abbreviation of atomic units.

Abbreviation of absorbance units.

A hypothetical world situated in a different dimension of space and time than the real world consisting of the universe known and experienced by human beings.

An imaginary realm, often a variant form of the real world, depicting a different way events could have unfolded or the universe could have functioned.

A unit of length equal to 10−10 meters (that is, one ten-billionth of a meter), approximately the size of an atom, and denoted by the symbol Å, used especially to measure the wavelength of electromagnetic radiation or distances between atoms.

149,597,870,700 metres, the approximate mean distance from the Earth to the Sun (the semi-major axis of Earth's orbit), (symbol AU), used to measure distances in the solar system.