Negative pressure is pressure less than that of the ambient atmosphere. Negative air pressure within a room will cause air to flow into the room from the outside whenever a door or window is opened. It is often used in the hospital setting to quarantine people afflicted with highly contageous, deadly, airborne diseases.
Negative pressure is also often found in sub-sea pipelines where the depth of water produces such a high external pressure that any leak would actually be from the sea water into the pipeline.
Negative pressure is the opposite of positive pressure.
Negative pressure is found in some theories of physics, such as those involving dark energy or negative mass. It results in a repulsive gravitational force. The Casimir effect produces a region of negative pressure.