Source : Webster's Revised Unabridged Dictionary (1913)

Agar-agar \A`gar-a"gar\, n.
   A gelatinlike substance, or a solution of it, prepared from
   certain seaweeds containing gelose, and used in the
   artificial cultivation of bacteria; -- often called {agar},
   by abbreviation.

Source : WordNet®

agar
     n 1: any culture medium that uses agar as the gelling agent [syn:
           {nutrient agar}]
     2: a colloidal extract of algae; used especially in culture
        media and as a gelling agent in foods [syn: {agar-agar}]