n :  the act or process of releasing from pressure or compression: as  a  :  reduction of pressure (1) :  the decrease of ambient air pressure experienced in an air lock on return to atmospheric pressure after a period of breathing compressed air (as in a diving bell or caisson) or experienced in ascent to a great altitude without a pressure suit or pressurized cabin (2) :  the decrease of water pressure experienced by a diver when ascending rapidly  b  :  an operation or technique used to relieve pressure upon an organ (as in fractures of the skull or spine) or within a hollow organ (as in intestinal obstruction) .
 
 
 
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