A slot is a receptacle or recess into which a piece can fit. The term may also refer to a position, such as an airline reservation or a slot on a runway or at an airport.

In a slot machine, players insert cash or paper tickets with barcodes into a designated slot and activate the machine by pulling a lever or pushing a button (either physical or on a touchscreen). The reels then spin and stop to rearrange the symbols and award credits according to a paytable. The symbols vary by machine, but classics include fruits, bells, and stylized lucky sevens. Some machines offer a progressive jackpot or bonus levels, which grow larger as more coins are played.

Charles Fey’s 1907 invention allowed automatic payouts and had three spinning reels, making it easier to win than the previous poker-themed machines. He also replaced the traditional poker card symbols with fruit and other themed icons, such as horseshoes, diamonds, spades, and hearts. Three aligned liberty bells were the highest winning combination and gave the machine its name.

Modern slot machines use microprocessors to determine the probability of a given symbol appearing on a particular pay line, which reduces the number of possible combinations by a cubic factor. They can also weight particular symbols, so that a given symbol might appear more frequently on a reel displayed to the player but less frequently overall on the multiple physical reels. This allows the manufacturer to advertise a higher chance of winning, although the actual probability is much lower.