The Collaborative International Dictionary of English v.0.48:

Casein \Ca"se*in\, n. [Cf. F. cas['e]ine, fr. L. caseur cheese.
   Cf. Cheese.] (Physiol. Chem.)
   A proteid substance present in both the animal and the
   vegetable kingdom. In the animal kingdom it is chiefly found
   in milk, and constitutes the main part of the curd separated
   by rennet; in the vegetable kingdom it is found more or less
   abundantly in the seeds of leguminous plants. Its reactions
   resemble those of alkali albumin. [Written also caseine.]