The Collaborative International Dictionary of English v.0.48:

Populin \Pop"u*lin\, n. [L. populus poplar: cf. F. populine.]
   (Chem.)
   A glycoside, related to salicin, found in the bark of certain
   species of the poplar (Populus), and extracted as a sweet
   white crystalline substance.
   [1913 Webster]