The "dew point" is the temperature where water vapor condenses into dew; it changes based on the amount of water in the air. When the dew point drops below the air temperature, you get dew.
When the dew point drops below freezing, it is called the "frost point"; when the air temperature drops below freezing and the dew/frost point drops below the air temperature, you get frost.
more about frost. I've been waiting for the first frost here but I don't think it has come yet.