Which gas or mixture is relative humidity defined for?

Relative humidity is a measure that describes the amount of water vapor present in the air relative to the maximum amount of water vapor the air can hold at a given temperature. It is expressed as a percentage. The concept inherently involves the gaseous mixture of air, which consists primarily of nitrogen, oxygen, and smaller amounts of other gases, including traces of carbon dioxide.

While other gases like carbon dioxide, nitrogen, and oxygen are significant in various contexts, relative humidity specifically pertains to the capacity of air to hold moisture. Therefore, the definition and measurements of relative humidity are applicable solely to atmospheres containing water vapor, which is predominantly found in air. This fundamental relationship makes air the correct answer, as it encompasses the entire gaseous mixture that interacts with water vapor. Understanding this context reinforces why air is the standard medium used to define and measure relative humidity in various scientific and industrial applications.