What is the definition of "astronomical unit"?

An astronomical unit is equal to 92.96 million miles or the mean distance between Earth and the sun. The International Astronomical Union refined the definition for this unit in 2012, making it a standard international value.

Prior to the redefinition, the AU measurement differed based on a number of constantly changing factors, including the time of day and mass of the sun. The AU originated in the 17th century, when astronomers used angles and trigonometry to express celestial distances, as they had no other way to make distance measurements. Initial calculations by Italian astronomer Giovanni Cassini calculated the AU to within 6 percent of its actual value.