Final answer:
Specific Gravity is the term that refers to the weight of a gas compared to air, representing the ratio of a substance's density to that of a reference material.
Step-by-step explanation:
The term that refers to the weight of a gas compared to air is Specific Gravity. Specific gravity is a dimensionless quantity that provides a comparison of the density of a substance to the density of a reference material, which is often water for liquids and solids, but can also be air for gases. This makes it a particularly useful property for understanding how a substance will behave in different environments, such as predicting whether an object will float or sink when placed in water, or if a gas will rise or be more dense than the surrounding air.