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 in a group, series, or sequence.

The earliest known slot machine was created by Sittman and Pitt in the 19th century. This contraption had five reels that displayed poker symbols and paid out winning combinations when three or more aligned poker cards appeared. But Charles Fey revolutionized the slot world with a new version that allowed automatic payouts, incorporated three reels instead of five, and used a more sophisticated system to display symbols. His first machine, dubbed the Liberty Bell, was a hit and inspired similar machines.

The term “slot” is also used to describe an allocated time for an aircraft to take off or land at an airport. Air traffic managers allocate slots to airlines based on the runway capacity and other considerations, with the goal of keeping the flow of air moving smoothly. Central flow management has resulted in huge savings for European airlines in terms of delays and fuel burn, and slots are expected to be deployed worldwide over the next 20 years.