While humidity can refer to the general presence of water vapor in the air, it's more often used to describe the amount of water vapor present. This can be expressed in different ways, such as:

* Relative humidity: The ratio of the actual amount of water vapor in the air to the maximum amount of water vapor the air can hold at a given temperature.

* Absolute humidity: The actual mass of water vapor per unit volume of air.

* Specific humidity: The mass of water vapor per unit mass of air.

So, while "water vapor" is the specific term for the gaseous form of water in the air, "humidity" is the more general term that encompasses the amount of water vapor present.