For research purposes, the Wikipedia entry on HUMan INTelligence in espionage.

HUMINT, a syllabic abbreviation of the words HUMan INTelligence, refers to intelligence gathering by means of interpersonal contact, as opposed to the more technical intelligence gathering disciplines such as SIGINT, IMINT and MASINT. NATO defines HUMINT as “a category of intelligence derived from information collected and provided by human sources.”[1] Typical HUMINT activities consist of interrogations and conversations with persons having access to pertinent information.