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]

WordNet (r) 3.0 (2006):

Cetraria islandica
    n 1: 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, Cetraria
         islandica]