A researcher was a person who conducted research in a particular field of endeavor.
Several researchers including Doctor Carol Marcus, David Marcus, Jedda, March, and Madison were assigned to the Federation research station Regula I. (Star Trek II: The Wrath of Khan)
In 2366, the anthropology researchers Palmer, Barron, and Mary Warren worked as cultural observers on Mintaka III. (TNG: "Who Watches The Watchers")