Dew point is the temperature to which air must be cooled to become saturated with water vapor, assuming constant air pressure and water content. When cooled below the dew point, moisture capacity is reduced and airborne water vapor will condense to form liquid water known as dew. When this occurs via contact with a colder surface, dew will form on that surface.The dew point is affected by humidity. When there is more moisture in the air, the dew point is higher.When the temperature is below the freezing point of water, the dew point is called the frost point, as frost is formed via deposition rather than condensation. In liquids, the analog to the dew point is the cloud point.

For example, the hydrogen dew point means the dew point of hydrogen,

Also known as dew point temperature, that is, under fixed air pressure, the gaseous water contained in the hydrogen reaches the temperature that it needs to be reduced to the liquid water. 

Air compression increases water vapor pressure and thus dew point. It is important to take this into consideration if you are bleeding the air to atmosphere before taking a measurement. The dew point at the measurement point will be different from the dew point in the process.

Dew point temperatures in compressed air range from ambient even down to -80 °C (-112 °F) in special cases. Compressor systems without air drying capability tend to produce compressed air that is saturated at ambient temperature. Systems with refrigerant dryers pass the compressed air through a cooled heat exchanger, causing water to condense out of the air stream. These systems typically produce air with a dew point no lower than 5 °C (41°F). Desiccant drying systems absorb water vapor from the air stream and can produce air with a dew point of -40 °C (-40 °F) and drier if required.

Dew Point of Molecular Sieve?