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Specific gravity is a dimensionless quantity that measures the ratio of the density of a substance to the density of a reference substance, typically water for liquids. When we refer to specific gravity in the context of liquids, it indicates how much heavier or lighter a liquid is compared to water when equal volumes are compared. If a liquid has a specific gravity greater than 1, it is denser than water and will sink; if it is less than 1, it is less dense and will float. This measurement is particularly important in firefighting and hazardous material handling, as it helps determine how liquids may behave in water and the potential for mixing, separation, or danger in case of spills or leaks.

The other options, while related to fluid and material properties, do not accurately define specific gravity. For instance, vapor density pertains to gases and compares the density of a gas to that of air, while the flash point refers to the lowest temperature at which a combustible material can vaporize to form an ignitable mixture in air. The surface to mass ratio typically relates to solids and their physical properties, which is not applicable to the measurement of specific gravity, which is focused on density comparisons.