It is computed from the specific gravity of the liquid at 60 °F: 141.5 divided by that specific gravity, minus 131.5. Fresh water is 10 degrees. A light crude sits well above water on the scale. A heavy crude sits closer to water, and an extra-heavy oil can fall below 10.
Stock-tank volumes, pump design, and the price a crude attracts all move with it, because it tracks density. It is not a viscosity. Two oils with the same gravity can flow very differently once they are full of wax or asphalt. A gravity quoted at a temperature other than 60 °F is a different number until it is corrected.