A slot machine
A slot machine was a gambling device often found in casinos.
Users of the device would deposit their wager, in an amount of money requested by the device, into it and then pull a lever located on the side. The machine would then display random and independent images on its front, and if those images formed various combinations, the user would be considered a winner and receive an amount of money proportional to the odds of the image sequence appearing. As with most forms of gambling, the odds were not in the user's favor.
Slot machines were described in the novel Hotel Royale, and were seen in a recreation of the Hotel, created for the benefit of Stephen G. Richey by an alien race. (TNG: "The Royale")
The casino within the Hotel Royale had several slot machines available for use, as did Vic's lounge within the Vic Fontaine holoprogram. (TNG: "The Royale", DS9: "It's Only a Paper Moon")
Kasidy Yates used a slot machine while trying to distract to the security guard guarding the count room. (DS9: "Badda-Bing, Badda-Bang")