The Collaborative International Dictionary of English v.0.48:

Carrageen \Car"ra*geen`\, Carrigeen \Car"ri*geen`\, n.
   A small, purplish, branching, cartilaginous seaweed
   (Chondrus crispus), which, when bleached, is the Irish
   moss of commerce. [Also written carragheen, carageen.]
   [1913 Webster] Carrageenan

WordNet (r) 3.0 (2006):

Chondrus crispus
    n 1: dark purple edible seaweed of the Atlantic coasts of Europe
         and North America [syn: Irish moss, carrageen,
         carageen, carragheen, Chondrus crispus]