The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]