A prostitute was one who trades sexual services for money or favor.
Upon first meeting Odo Kira Nerys mistakenly thought that Odo was attempting to purchase sexual services from her, to which she remarked that she did not do that either for money or food. (DS9 episode: "Necessary Evil")
Due to the state of disrepair that Deep Space 9 was in when Starfleet first began administering the station, Chief Miles O'Brien spent a great deal of time doing repairs. This led Quark to remark that there were prostitutes that spent less time on their backs than O'Brien did. (DS9 novel: The Siege)