informant (n.)

1690s, "someone who supplies information," from Latin informantem (nominative informans), present participle of informare "train, instruct, educate" (see inform). Occasionally as "one who gives information to the authorities, one who dishonorably betrays knowledge gained in confidence" (1783). Informer is older in both senses and more usual in the latter. As an adjective from 1890.