A tuxedo was a type of formal suit worn mainly by Humans during the 20th century. A tuxedo usually consisted of matching coat and pants, typically black, a white shirt, and a bow tie. They were considered very formal attire and normally only worn on exceptional occasions.
Julian Bashir often wore a tuxedo in his role as the holographic character Patrick Merriweather. (DS9: "Our Man Bashir", "A Simple Investigation", "Change of Heart")
Elim Garak wore a tuxedo when he went into the holosuite to see Bashir's new program for the first time. (DS9: "Our Man Bashir")
Jean-Luc Picard appeared in a tuxedo for the holoprogram The Big Good-Bye in order to gain entrance to an exclusive club. (Star Trek: First Contact)
Odo assumed the appearance of wearing a tuxedo when he was in Vic Fontaine's lounge. He later morphed his clothing into a tuxedo shortly before rejoining the Great Link, as Kira Nerys thought he looked his best while wearing one. (DS9: "His Way", "The Sound of Her Voice", "Badda-Bing, Badda-Bang", "What You Leave Behind")
Nog wore a tuxedo while he was living in the holosuite and working in the lounge. Benjamin Sisko later wore one when he helped to rid the Bashir 62 holoprogram of Frank "Frankie Eyes" Chalmers. (DS9: "It's Only a Paper Moon", "Badda-Bing, Badda-Bang")
The Doctor wore a tuxedo during his farewell musical performance on the Qomar homeworld in 2376. (VOY: "Virtuoso")
This tuxedo, worn by Robert Picardo, was later sold off on the It's A Wrap! sale and auction on eBay. [1](X)
A stage direction in the final draft script of the Star Trek: The Original Series episode "Miri" stated that some Only boys, in that episode, were wearing tuxedos, though none of the boys are shown wearing tuxedos in the final version of that episode.