[AIVC, Air infiltration and ventilation glossary, 1992]:the difference in pressure across a building envelope or component whether caused by natural or artificial means.

(See internal pressure distribution)

[EN 16798-3:2017, 3.14]:design load condition for pressure drop are: Average of clean filters and recommended maximum pressure drop and for other components (heat exchanger, cooling coil, humidifier, etc.) the average of dry and wet values.