A slot is a narrow opening or groove in something. You can put things like letters and postcards through a slot in a door or wall. A slot can also refer to a position or time, as in a slot on the subway or a schedule slot for an appointment. Other uses include a position in an organization or club, a seat on a bus or airplane, or a berth on a ship or submarine. The word is derived from the Latin slot, meaning hole, slit, or aperture.

Historically, all slot machines used revolving mechanical reels to display and determine results. Three physical reels allowed only 103 = 1,000 possible combinations, so jackpot sizes were limited. In the 1980s manufacturers began incorporating electronics into their machines and programming them to “weight” certain symbols, making it more likely that they would appear on a given payline.

Modern slot machines accept cash or, in “ticket-in, ticket-out” machines, paper tickets with barcodes that are scanned for validation. When a player activates a machine by pressing a lever or button (physical or on a touchscreen), the reels spin and stop to rearrange the symbols, paying out credits based on a paytable. Most slot games have a theme, and symbols and other bonus features are aligned with that theme. Some slots are themed after famous movies, TV shows, or cities; others have more abstract themes like science fiction or adventure. Despite the popularity of these machines, forces of morality and religion frequently oppose their operation, leading to legislation restricting their sale and transportation or prohibiting them altogether except in private social clubs.