Source : Webster's Revised Unabridged Dictionary (1913)

Informer \In*form"er\, n. [From {Inform}, v.]
   1. One who informs, animates, or inspires. [Obs.] --Thomson.

            Nature, informer of the poet's art.   --Pope.

   2. One who informs, or imparts knowledge or news.

   3. (Law) One who informs a magistrate of violations of law;
      one who informs against another for violation of some law
      or penal statute.

   {Common informer} (Law), one who habitually gives information
      of the violation of penal statutes, with a view to a
      prosecution therefor. --Bouvier. Wharton.

Source : WordNet®

informer
     n : one who reveals confidential information in return for money
         [syn: {betrayer}, {rat}, {squealer}, {blabber}]