How do we define dewpoint?

Dewpoint is defined as the temperature to which a gas (e.g. air) must be cooled, at constant pressure, for water vapour to begin to condense to liquid water. In other words, when the dewpoint temperature has been reached, the gas is fully saturated with water vapour. The term “pressure dewpoint” refers to the dewpoint temperature of a gas at pressures higher than atmospheric pressure. When addressing dewpoint in pressurized compressed air, the correct terminology is actually “pressure dewpoint,” but this is often shortened to “dewpoint” in common usage.