A comedian or comic was someone who can make people laugh, either as a professional or an amateur.
Professional comedians were often found warming-up the audience for the main act, or were the main acts themselves in comedy clubs. Charnock's Comedy Cabaret was one such example. (TNG: "The Outrageous Okona")
In 2375, Vic Fontaine was looking for an opening act for his lounge. Rom attempted to apply for the position as a singer, but after hearing Rom's singing talent, Vic noted that what he was really looking for was a comedian. He added that having two singers on the same bill was a one-way ticket to deadsville. (DS9: "The Siege of AR-558")