Negative pressure is a common term when referring to differential pressure. The place I most commonly see it is for isolating rooms with positive[1] or negative[2] pressure relative to their surroundings.

[1] https://en.wikipedia.org/wiki/Positive_pressure

[2] https://en.wikipedia.org/wiki/Negative_room_pressure

"Pressure" in common usage typically means "gauge pressure", which is zero-referenced to ambient air pressure.