c. 1200, professioun, "vows taken upon entering a religious order," from Old French profession (12c.) and directly from Latin professionem (nominative professio) "public declaration," noun of action from past-participle stem of profiteri "declare openly" (see profess).
The meaning "any solemn declaration" is from mid-14c. Meaning "occupation one professes to be skilled in, a calling" is from early 15c.; meaning "body of persons engaged in some occupation" is from 1610; as a euphemism for "prostitution" (compare oldest profession) it is recorded from 1888.
Formerly theology, law, and medicine were specifically known as the professions; but, as the applications of science and learning are extended to other departments of affairs, other vocations also receive the name. The word implies professed attainments in special knowledge, as distinguished from mere skill; a practical dealing with affairs, as distinguished from mere study or investigation; and an application of such knowledge to uses for others as a vocation, as distinguished from its pursuit for one's own purposes. In professions strictly so called a preliminary examination as to qualifications is usually demanded by law or usage, and a license or other official authority founded thereon required. [Century Dictionary]