negative pressure

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Pressure lower than a given reference pressure, generally causing suction rather than exerting an outward pushing force.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Pressure that is less than normal atmospheric pressure
• noun The tension in a liquid that is thermodynamically metastable with respect to the vapour phase; especially that in the xylem of vascular plants that allows transpiration to great heights