The Collaborative International Dictionary of English v.0.48:

Barbituric acid \Bar`bi*tu"ric ac"id\ (Chem.)
   A white, crystalline substance, CH2(CO.NH)2.CO, derived
   from alloxantin, also from malonic acid and urea, and
   regarded as a substituted urea.
   [1913 Webster] Barbizon school

WordNet (r) 3.0 (2006):

barbituric acid
    n 1: a white crystalline acid derived from pyrimidine; used in
         preparing barbiturate drugs [syn: barbituric acid,
         malonylurea]