The Collaborative International Dictionary of English v.0.48:

niacin \niacin\ n.
   A B vitamin (vitamin B5) essential for the normal function of
   the nervous system and the gastrointestinal tract. Called
   also nicotinic acid and antipellagra vitamin. Chemical
   formula C6H5NO2, chemically it is 3-pyridinecarboxylic
   acid.

   Syn: nicotinic acid.
        [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

nicotinic acid \nic`o*tin"ic ac"id\, n. (Chem.)
   An organic acid (C6H5NO2), a carboxylated derivative of
   pyridine, obtained by the oxidation of nicotine; called also
   niacin and antipellagra vitamin, and vitamin B5.
   Chemically it is 3-pyridinecarboxylic acid.

   Note: It is a cofactor in several enzymatic reactions, and is
         used to prevent a pellagra-like disease in dogs.
         --[MI11]

   Syn: 3-pyridinecarboxylic acid, niacin, antipellagra vitamin.
        [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

nicotinic acid
    n 1: a B vitamin essential for the normal function of the
         nervous system and the gastrointestinal tract [syn:
         niacin, nicotinic acid]