A advertisement was a type of communication that involved a message of some sort intended to promote a product or a service, thus attracting new customers.
In 2372, Quark hacked into the communications systems aboard Deep Space 9, by using an encryption program to bypass the access protocols, and placed a short visual sequence promoting his bar. This was considered a class 3 offense according to Odo. As part of the same campaign, Quark had also tampered with the replicators aboard the station, such that beverages appeared in mugs advertising the bar, much to Worf's annoyance. Both the video and the mug included a jingle in the form of a simple song, sung by Quark, encouraging potential customers to visit his place. (DS9: "The Quickening")
Individuals aboard vessels visiting Freecloud were greeted by holographic advertisements for establishments, such as Feely's Venom Garden and The Red Bolian. One had to interact with the advertisements in some way to make them go away. (PIC: "Stardust City Rag")