Condensation

Condensation is the change in matter of a substance to a denser phase, such as a gas (or vapour) to a liquid.

Condensate

Liquid which has been condensed from a vapour is called condensate.

Dew point

Dew point is the temperature at which water vapour reverts from its gas state to a liquid. Sometimes referred to as dew point temperature.

Or for the purists:

The dew point of a given parcel of air is the temperature to which the parcel must be cooled at constant barometric pressure, for the water vapour component to condense into water.

Relative Humidity

Relative humidity is the term used to describe the amount of water vapour in the atmosphere at a given temperature expressed as a percentage of the maximum possible at that temperature.

Or for the purists:

Relative humidity is defined as the ratio of the partial pressure of water vapour in a gaseous mixture of air and water to the saturated vapour pressure of water at a given temperature. Relative humidity is expressed as a percentage.

Vapour

Vapour is the gaseous state of matter that is a solid or liquid at normal room temperature.

Vapour Pressure

The balance between condensation and evaporation gives the quantity called vapour partial pressure which is abbreviated to Vapour Pressure. Vapour Pressure is an indication of a liquid's evaporation rate. It relates to the tendency of molecules and atoms to escape from a liquid or a solid. A substance with a high vapour pressure at normal temperatures is often referred to as volatile an example being petrol. The higher the vapour pressure of a material at a given temperature, the lower its boiling point.

Water Vapour

Water vapour is the gas phase of water.