A slot is a narrow opening or groove. In computerized slot machines, the microprocessors inside give each symbol on a reel a different probability of appearing. This means that the appearance of a particular symbol does not necessarily mean that the machine has paid out. In addition, modern slot machines allow players to wager on multiple lines simultaneously, which can run up, down, sideways, or diagonally. Each line requires a separate coin or paper ticket with a barcode to activate, and each symbol that appears on a winning line wins credits based on the paytable.

The appeal of slots lies in their simplicity and the instant feedback they provide. Unlike traditional casino games, which often require lengthy periods of time between spins and provide little information about the outcome of a game, slots offer fast, predictable results accompanied by high-fidelity attention-grabbing music and amusing animations. In addition, winning a slot machine is often accompanied by a gratifying sound or light that signals the receipt of the payout.

In computer science, a slot is a dynamic placeholder on a Web page that either waits for content to be added (a passive slot) or that calls out to a renderer to fill it in (a active slot). It is a more general use case for encapsulating reusable logic and visual output, than the scoped slots discussed above.