The Collaborative International Dictionary of English v.0.48:

Informer \In*form"er\ ([i^]n*f[^o]rm"[~e]r), n. [From Inform,
   v.]
   1. One who informs, animates, or inspires. [Obs.] --Thomson.
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            Nature, informer of the poet's art.   --Pope.
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   2. One who informs, or imparts knowledge or news.
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   3. (Law) One who informs a magistrate of violations of law;
      one who informs against another for violation of some law
      or penal statute.
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   Common informer (Law), one who habitually gives information
      of the violation of penal statutes, with a view to a
      prosecution therefor. --Bouvier. --Wharton.
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Bouvier's Law Dictionary, Revised 6th Ed (1856):

COMMON INFORMER. One who, without being specially required by law, or by
virtue of his office, gives information of crimes, offences or misdemeanors,
which have been committed, in order to prosecute the offenders; a
prosecutor. Vide Informer; Prosecutor.