A professional was an individual who was a member of a profession, where professionalism was the competence or skill expected of a professional.
In 2265, after Gary Mitchell said he was sorry for calling Doctor Elizabeth Dehner a "walking freezer unit", she said that women professionals tended to overcompensate. (TOS: "Where No Man Has Gone Before")
During a tense siege aboard a Jem'Hadar attack ship crashed on Torga IV, Benjamin Sisko admonished his team for not acting like the professionals they were. (DS9: "The Ship")