A slit or narrow opening, especially one for receiving something such as a coin or a letter. Also: a position, as on the team or in an assignment; a slot on a track or rink.

In modern gambling machines, a person inserts cash or paper tickets with barcodes into a slot and activates a mechanism that spins reels to rearrange the symbols. When a winning combination is lined up, the player earns credits according to the pay table. Some slot games have wild symbols that can substitute for other symbols to complete a line. Depending on the theme, symbols may include objects such as fruits, bells, or stylized lucky sevens.

With microprocessors now ubiquitous, slot manufacturers have incorporated electronics into their machines and programmed them to weight particular symbols differently. This makes it appear that a particular symbol is more likely to appear on a given reel than it actually is. Thus, the 15-coin payout on a three-reel machine may seem like a jackpot, whereas in reality it is only a medium prize.

In cryptography, a slot is a time or block that can be used for building blocks. Slots can be empty if no delegate wins the leadership for that slot and fails to construct a block within a certain time limit. In the Ouroboros Praos algorithm (which builds on earlier work by the IOHK team), each slot is assigned to a single leader who randomly selects a block to build in that slot.