Informer /(ĭn*fôrm"ẽr)/

In·form·er

Informer

n.
  1. One who informs, animates, or inspires. [Obs.]
    Nature, informer of the poet's art.
  2. One who informs, or imparts knowledge or news.
  3. One who informs a magistrate of violations of law; one who informs against another for violation of some law or penal statute. (Law)

Phrases & Compounds
Common informer
one who habitually gives information of the violation of penal statutes, with a view to a prosecution therefor.