The Collaborative International Dictionary of English v.0.48:

Butyrin \Bu"ty*rin\, n. (Physiol. Chem.)
   A butyrate of glycerin; a fat contained in small quantity in
   milk, which helps to give to butter its peculiar flavor.
   [1913 Webster]

WordNet (r) 3.0 (2006):

butyrin
    n 1: any of three glycerides of butyric acid