informant

tietolähde

informantti One who relays confidential information to someone, especially to the police; an informer.
puhekieltä A native speaker who acts as a linguistic reference for a language being studied. The informant demonstrates native pronunciation, provides grammaticality judgments regarding linguistic well-formedness, and may also explain cultural references and other important contextual information.
1977, A. E. Kibrik, The methodology of field investigations in linguistics

The only material the linguist has to begin with are the informant's grammatical utterances in the target language pronounced arbitrarily in a natural or assigned communicative situation or stimulated artificially by the investigator.

The informant learns his language by formal training and, more importantly, by constant exposure to its use. He cannot repeat to the linguist what he has never seen or heard.