The Collaborative International Dictionary of English v.0.48:

Agar-agar \A`gar-a"gar\, n. [Ceylonese local name.]
   1. A fucus or seaweed much used in the East for soups and
      jellies; Ceylon moss (Gracilaria lichenoides).
      [1913 Webster]

   2. A gelatinlike substance, or a solution of it, prepared
      from certain seaweeds containing gelose (such as Ceylon
      moss, Gracilaria lichenoides or other seaweeds of the
      genera Gelidium, Ceramium, Pterocladia, and
      Eucheuma), and used for solidifying growth media in the
      artificial cultivation of bacteria, or as a gelling agent
      in foods; -- usually called simply agar, by
      abbreviation.

   Note: In composition it is predominantly a polysaccharide,
         and is not degraded by most bacteria. It thus almost
         completely replaced the earlier protein-based gelatins
         used for fixing bacterial colonies on culture plates,
         as the gelatins were often dissolved by the proteolytic
         enzymes common in bacteria.

   Syn: gelose, agar.
        [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

agar-agar
    n 1: a colloidal extract of algae; used especially in culture
         media and as a gelling agent in foods [syn: agar, agar-
         agar]