A councilor is a member of a council, an assembly of persons formed for deliberation or action: "Jesse was appointed a member of the Mayor's Council on Mass Transportation." A counselor is one who gives advice or counsel (opinion, instruction): "Martha acted as a counsel to the Committee on Welfare." The corresponding verb form is counsel: "Please counsel me in this matter." Counselor is also a term applied to lawyers, although among attorneys the word is often employed humorously or sarcastically. Certain employees at summer camps are called counselors.
Dictionary of problem words and expressions. Harry Shaw. 1975.