A slot is a narrow, elongated depression, groove, or notch, especially one for receiving or admitting something, such as a coin or a letter. In linguistics, a slot is a position in a construction into which any of a number of morphemes may fit.

A modern slot machine is a computer-controlled device that uses a random number generator (RNG) to determine the outcome of a spin. The RNG generates thousands of numbers per second, and each combination of symbols that appear on a reel corresponds to a different probability of winning or losing. In addition, most slot machines offer a Wild symbol that acts as a substitute for other symbols, and in some cases, winning combinations can open bonus levels or jackpot levels.

The first slot machines were invented in the 19th century by Sittman and Pitt, who used five drums with a total of 50 poker cards to create a machine where players could win by lining up poker hands. Charles Fey improved on this design, introducing three reels and substituting the poker symbols with hearts, horseshoes, diamonds, and Liberty bells. He also added an automatic payout system and a lever that operated the hopper door.

Many online casinos publish payback percentages for their slot games. However, these percentages are often misleading and vary from game to game. In some cases, the payback percentages are calculated using a specific set of rules and assumptions, which can be misleading for players. Other casinos use a different formula, calculating the return on investment based on a number of variables, including time spent playing the game.