@wikipedia

Synonym: Relative Density = Specific gravity

Dimensionless ratio of the actual fluid density to the fluid density of a reference substance , both taken at reference conditions (equal or close to STP, which should be specified individually):

SG= \frac{\rho}{\rho_{\rm ref}}

The selection of reference substance and the corresponding value of depends on the context.

For fluids which are liquid at STP this is usually a distilled water and for fluids which are gaseous at STP this is usually a dry air:

Liquid specific gravity	Gas specific gravity

See also