What is an astronomical unit? why is this unit used to measure distances in the solar system?

An astronomical unit is a unit of measurement equal to 149.6 kilometers, the mean distance from the center of the earth to the center of the sun.

1 astronomical unit = 92955807.3 miles

A astronomical unit is to measure light years.

Atomicity occurs to prevent updates of the database occurring only partially, which can cause greater problems than rejecting the whole series outright.some are monatomic, diatomic, triatomic etc because it shows the number of atoms. for example, monatomic means it contains only one atom.

Ithinks its false. tell me if im wrong