What is Informer?

Legal Definition
A person who informs or prefers an accusation against another, whom he suspects of the violation of some penal statute.

2. When the informer is entitled to the penalty or part of the penalty, upon the conviction of an offender, he is or is not a competent witness, accordingly as the statute creating the penalty has or has not made him so. 1 Phil. Ev. 97; Rosc. Cr. Ev. 107; 5 Mass. R. 57; 1 Dall. 68; 1 Saund. 262, c. Vide articles Prosecutor; Rewards.
-- Bouviers Law Dictionary
Legal Definition
A person who informs or prefers an accusation against another, whom he suspects of the violation of some penal statute. See Common informer.
-- Black's Law Dictionary
Legal Definition
One who furnishes information for the prosecution of a crime. See, also, Common informer.
-- Ballentine's Law Dictionary