Definitions for "Partial pressure"
This is the pressure created by one component of gas mixture. The partial pressure of fuel vapor in a well-mixed ullage over a liquid fuel layer is equal to the vapor pressure of the liquid under those conditions. Strictly speaking, this is the case only for a single component fuel (such as hexane) but is also valid for multicomponent fuels like kerosene as long as sufficient liquid is present.
The fraction of the total pressure of a mixture of gases that is due to one component of the mixture.
pressure exerted by a gas if it were the only gas in a container