ə̇nˈfȯrmər, -ȯ(ə)mə(r) noun
Etymology: Middle English enfourmer, from enfourmen to inform + -er — more at inform
1. obsolete : one that informs, animates, or inspires
nature, informer of the poet's art — Alexander Pope
2. : one that informs or imparts knowledge or news
3. : one that informs against another:
a. : one that informs a magistrate of a violation of law : one that lays an information ; especially : one that makes a practice of informing against others for violations of penal laws particularly when the informer may receive as a reward a share of the money penalty imposed — called also common informer ; compare qui tam
b. : one secretly in the service of the police or of a diplomatic agency (as an embassy) that supplies information
a nest of spies and informers