Source : Webster's Revised Unabridged Dictionary (1913)

Cetrarin \Cet"ra*rin\, n. [From Cetraria Islandica, the
   scientific name of Iceland moss.] (Chem.)
   A white substance extracted from the lichen, Iceland moss
   ({Cetraria Islandica}). It consists of several ingredients,
   among which is cetraric acid, a white, crystalline, bitter
   substance.

Source : WordNet®

Cetraria islandica
     n : lichen with branched flattened partly erect thallus that
         grows in mountainous and arctic regions; used as a
         medicine or food for humans and livestock; a source of
         glycerol [syn: {Iceland moss}, {Iceland lichen}]