In a casino, a slot machine is a mechanical game with reels and a paytable. Players insert cash or paper tickets with barcodes and activate the machine, which spins the reels and pays out credits when combinations of symbols match the symbols on the paytable. The symbols on a slot machine vary based on theme, but classic symbols include fruit, bells, and stylized lucky sevens. Modern slots often have bonus features that align with the theme.

The word slot is derived from the Greek slit, or narrow opening, or from the Latin word slit. This word reflects the use of slots in various contexts, including the narrow opening in the copy desk where the chief copy editor sits. The word slot is a slit or depression, and can refer to a wide variety of objects. For example, a slot in an airplane wing may be a groove or notch. In the ice hockey world, a slot is a rimless unmarked area between the two face-off circles.

There are numerous types of slot machines on the market, and the payout percentages vary depending on the type of machine. Most manufacturers will use a fixed payback percentage, but the payout percentage can be altered by the manufacturer. The payout percentage of a slot machine may be set loosely, and this can affect how much a player wins, or loses. The payback percentage of a slot machine is usually set at a certain percentage, typically between seventy to ninety percent. However, any payback percentage below 100 is considered a win for the casino.