Answer
It's the ratio of the density of a substance to the density of a reference substance.
Work Step by Step
The specific gravity it's the ratio of the density of a substance to the density of a reference substance, usually water at $4^{\circ}C$ and 1 atm for liquids or air at $25 ^{\circ}C$ and 1 atm for gases.
$ SG = \frac{d}{d_{ref}} $
