A narrow notch, groove, or opening, as in a keyway in machinery or a slit for a coin in a vending machine. Also: a position, as in a group, series, or sequence: He had an eight-o’clock slot on the TV show. A place for something: He slipped the disc into the slot in the dashboard.

A space in a computer’s memory or disk in which a file can be stored. In computer games, a slot is a location in the game’s interface that allows the player to select a specific action.

In aviation, a time allocation for the departure or arrival of an aircraft at an airport during a particular period of the day, typically limited to prevent repeated delays caused by too many flights trying to take off and land at the same time. Airlines often buy and sell slots. An airline that holds a slot can operate the aircraft at the allocated times without having to wait for clearance from air traffic control.

The first slot machines were invented in the 19th century. Sittman and Pitt made the first one in 1891, which had five drums and 50 poker symbols. The earliest known slot machines were in bowling alleys, but by the 1860s they could be found everywhere. Charles Fey improved upon the original design, creating a more sophisticated machine with five reels and 20 symbols that would earn the player credits when certain combinations line up on the payline. It was the first of what we now call modern slot machines, and it became an instant success.

Modern slot machines use microprocessors to give different probabilities to each symbol on each reel. This means that the same symbol might appear on multiple stops on a reel, and to the player, seem close to a winning combination, but the odds are still much lower than they would be with a physical reel. This has also allowed manufacturers to weight symbols, so that some may appear more frequently than others.

Many slot machines have a specific theme, such as an ocean voyage or a western town. Symbols vary according to the theme and can include classic objects such as fruit, bells, and stylized lucky sevens. Most have a pay table, which shows the number of credits players will receive when the symbols listed on the table line up on the machine’s payline. The pay table can be printed on the machine or, in “ticket-in, ticket-out” machines, displayed on a video screen.

Some researchers have suggested that people who play slot machines develop gambling addictions more quickly than those who gamble on traditional casino tables, although the evidence is not conclusive. Psychologists have also studied how people engage with slot machines, discovering that they reach a debilitating level of involvement three times faster than people who play other types of gambling games. They also tend to gamble more money per session. For these reasons, slot machines are regulated by state governments.