A slot is a dynamic placeholder on a Web page that either waits for content to be added (a passive slot) or calls out to a renderer to fill it in. A slot can also be a component on a motherboard, used to house expansion cards such as an ISA, PCI, or AGP slots.

In a slot machine, the player inserts cash or, in “ticket-in, ticket-out” machines, a paper ticket with a barcode, into an opening on the face of the machine. The machine then displays reels with symbols and pays out credits based on the paytable when the symbols line up with the winning combination. Most slot games have a theme, and the symbols and bonus features are aligned with that theme.

The popularity of video slots has created concerns that they may lead to gambling addiction. Psychologists have found that players who play slot machines reach debilitating levels of involvement with gambling three times as quickly as those who play other casino games. This is despite the fact that video slots provide only intermittent rewards. Research indicates that this rapid escalation is due to the psychological arousal that accompanies the anticipation of receiving a reward, and the attention-capturing, intermittent nature of the reward. Other factors, such as the hedonic and utilitarian dimensions of slot enjoyment, are also related to reward reactivity. (See dark flow).