A commercial as seen on Paris and Torres' TV set
Tribbles Cereal Commerical "...Tribbles."
A commercial was a promotional medium used in television or radio to sell products or services, subsidizing the programming the audience was enjoying.
A commercial was once made advertising a number of different kinds of "Tribbles" cereals, though the context in which the commercial was made is uncertain. (ST: "The Trouble with Edward")
The commercial for "Tribbles" was presented as a post-credits scene in ST: "The Trouble with Edward". The context of the commercial from the in-universe perspective is impossible to determine, and may not have been meant to be taken literally.
Paid commercials were used by Empire TV on planet 892-IV in 2268, broadcast during highly popular programs such as Name the Winner. (TOS: "Bread and Circuses")
Bang Bang Automatic Rifles advertised its products on a radio station on Sigma Iotia II in the same year. (TOS: "A Piece of the Action")
A real estate commercial falsely advertised property on "beautiful Nimbus III" in 2287. (Star Trek V: The Final Frontier)
When B'Elanna Torres recreated a TV set for Tom Paris in 2376, she didn't understand the purpose of a jingle, a song designed to add appeal to a commercial. (VOY: "Memorial")
On Ferenginar, commercials for Slug-o-Cola were constant during Ferengi shows such as Pog & Dar: Cop Landlords and Will They, Won't They?. However, it was not limited to the commercial breaks, as the scripts of the shows were developed in such a way that the advertising of the soft drink was written into the story of the show itself, and not merely by means of product placement. Brad Boimler had apparently never seen a commercial as he seemingly reacted to it for the first time in amusement, calling it a "commer-seal". He also claimed that it was like mind control. (LD: "Parth Ferengi's Heart Place")