What term describes the density of gases in relation to air?

Vapor density specifically refers to the density of a gas compared to that of air. It is a crucial metric in understanding how gases will behave when released into the environment. For instance, a vapor density less than 1 indicates that the gas will rise and disperse in the air, while a vapor density greater than 1 means that the gas will tend to sink and accumulate near the ground. This property is significant in fire behavior and safety assessments, as the behavior of vapors can influence fire spread and potential hazards.

The other options, while related to density or gases, do not specifically denote the relationship of a gas's density in comparison to air.