A narrow depression, hole, or aperture.
Slot is an evocative word, with many associations: from the slit in a typewriter to the cut in a door, or the apex of a wedge-shaped building. It can also refer to a position or period in a sequence or schedule: She booked her haircut for the 2 p.m. slot. To slot something into another object or place is to fit it snugly and easily, as when the seat belt in a car slots into the buckle.
In casino games, a slot machine is a mechanical device with reels and symbols that spin when the player activates it by pressing a lever or button (either physical or virtual). The symbols land randomly on a payline to form winning combinations, and the player earns credits according to the paytable. Slot machines are typically themed, and their symbols and bonus features reflect that theme.
The first slot machines were designed to be completely random, but since the advent of microprocessors they have incorporated more complicated probability calculations. For example, manufacturers weight each reel differently, so that it is less likely to hit the higher-paying symbols early in a game than later. This can create an illusion that a jackpot is close to being won, when in fact the chances are much lower. This is one way casinos try to keep players engaged, and the success of these strategies depends on how well they can predict which reels will land the symbols they need.