The Collaborative International Dictionary of English v.0.48:

Alizarin \A*liz"a*rin\, Alizarine \A*liz"a*rine\, n. [F.
   alizarine, fr. alizari.] (Chem.)
   A coloring principle, C14H6O2(OH)2, found in madder, and
   now produced artificially as an an orange-red crystalline
   compound from anthracene. It is used in making red pigments
   (such as the Turkish reds), and in dyeing.
   [1913 Webster WordNet 1.5]

WordNet (r) 3.0 (2006):

alizarin
    n 1: an orange-red crystalline compound used in making red
         pigments and in dyeing [syn: alizarin, alizarine]