Final answer:
When air is at its maximum capacity for holding water vapor, it is considered saturated. This occurs at 100% relative humidity, representing a balance between the vapor pressure of water and the partial pressure of water in the air.
Step-by-step explanation:
When air holds all the water vapor it possibly can, we say it's saturated. This condition corresponds to a relative humidity of 100%, indicating that the air's capacity to contain water vapor has reached its limit at the given temperature. Evaporation is greatly inhibited at saturation because the air is equal to the vapor pressure of water, and no additional water can transition into the vapor phase.
This state often occurs at the dew point, at which water vapor begins to condense, potentially forming fog. It's important to note that warmer air can contain more water vapor, which is why devices like hair dryers use hot air to effectively promote evaporation and drying.