The Collaborative International Dictionary of English v.0.48:

Carrageenan \Car`ra*geen"an\, Carrigeenin \Car`ri*geen"in\, n.
   a colloidal material obtained from seaweed or Irish moss,
   used as an thickening or emulsifying agent and for
   stabilizing foods, cosmetics, and pharmaceuticals.
   [PJC]

WordNet (r) 3.0 (2006):

carrageenan
    n 1: a colloidal extract from carrageen seaweed and other red
         algae [syn: carrageenin, carrageenan]