Backpressure: The pressure that exists at the outlet of a pressure relief device as a result of flow in the discharge system.  Backpressure can be either constant or variable.  Backpressure is the sum of the built-up and superimposed backpressures.

Built-up Backpressure: The increase in pressure in the discharge header that develops as a result of flow after the pressure relief device or devices open up.