A con artist or confidence man, also known as a flim-flam man or flim-flam artist, was a person who gained another's confidence through trickery, then exploited that trust for his or her own gain. This was a form of fraud. A hustler was another type of con artist.
Harry Mudd was a notorious con artist in the late 23rd century. (TOS: "Mudd's Women", "I, Mudd"; TAS: "Mudd's Passion")
Captain Jean-Luc Picard once referred to Q as a "flim flam man". Worf, however, did not understand the term. (TNG: "Hide And Q")
In 2367, a female con artist attempted to trick the inhabitants of Ventax II into believing that she was Ardra, an equivalent of the Devil. As the crew of the USS Enterprise-D became embroiled in her scheme, Picard told Data that he had been examining the dynamics of what had once been known as the "con game." He described it as a fascinating subject, worthy of his study. (TNG: "Devil's Due")
Quark told Odo that he would have every confidence man aboard Deep Space 9 by the next day after he heard that Odo was temporarily relieved from duty. (DS9: "A Man Alone")
Martus Mazur was an El-Aurian con artist who used his natural powers to con a variety of people on Deep Space 9 in 2370, including Alsia and Roana. Quark saw past his charms and realized he was a con artist. (DS9: "Rivals")
Odo's official report on Quark described the Ferengi as "a self-important con artist who's nowhere near as clever as he thinks he is." (DS9: "Civil Defense")
The Ferengi Arridor and Kol conned the people of a primitive Delta Quadrant planet into thinking that the two Ferengi were the Holy Sages, demigods spoken of in the epic poem Song of the Sages. The crew of the USS Voyager put a stop to this after Commander Chakotay and Lieutenant Tom Paris discovered the exploitation while investigating the possible presence of a wormhole to the Alpha Quadrant, as well as the signature of a replicator from the Alpha Quadrant. (VOY: "False Profits")
Dala, Mobar, and Zar were con artists who in 2376 impersonated Captain Kathryn Janeway, Lieutenant Commander Tuvok, and Commander Chakotay, respectively, in a string of fly-by-night scams. (VOY: "Live Fast and Prosper")
In 2377 Gar, a con artist and swindler, stole The Doctor's hologram. (VOY: "Critical Care")