API Gravity
API (American Petroleum Institute) gravity is an arbitrary measurement of the relative density of petroleum liquids. There is an inverse relationship between API gravity and density: the lower the API gravity the higher the density of the liquid. API gravity has no units but is often referred to as being in degrees. It serves as an approximate measure of the relative value of a petroleum liquid. The higher the API gravity of a crude oil the richer the yield in high value refined oil products such as gasoline. Crudes with an API gravity of more than 30 are said to be light crudes, those of less than 20 are said to be heavy crudes. Heavy crudes yield a greater proportion of lower priced oil products such as fuel oil.
See also: http://www.api.org/, Brent, Dubai Crude, WTI