A commercial is a promotional medium used in television or radio to sell products or services, subsidizing the programming the audience is enjoying.
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")