Gigametre

The gigametre (official spelling used by the International Bureau of Weights and Measures; SI symbol: Gm) or gigameter (American spelling) is a unit of length in the metric system, equal to one billion metres, the SI base unit of length. This is equivalent to 1,000,000 km or approximately 621,370 mi.

Gigametres (from the Greek words gigas = giant and metro = count/measure) are composed exactly of 1,000 megametres. Although fit for distances in astronomy (e.g. a planet's distance from its star) this professional field usually uses the astronomical unit (AU) or millions and billions of kilometres.