n 1  :  a crystallizable protease that in an acid medium digests most proteins to polypeptides (as by dissolving coagulated egg albumin or causing casein to precipitate from skim milk), that is secreted by glands in the mucous membrane of the stomach of higher animals, and that in combination with dilute hydrochloric acid is the chief active principle of gastric juice  2  :  a preparation containing pepsin obtained as a powder or scales from the stomach esp. of the hog and used esp. as a digestant .
 
 
 
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