n 1  :  the act or an instance of inhaling : specif  :  the action of drawing air into the lungs by means of a complex of essentially reflex actions that involve changes in the diaphragm and in muscles of the abdomen and thorax which cause enlargement of the chest cavity and lungs resulting in production of relatively negative pressure within the lungs so that air flows in until the pressure is restored to equality with that of the atmosphere  2  :  material (as medication) to be taken in by inhaling in*ha*la*tion*al adj 
 
   
 
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