A slot is a narrow opening or groove in a surface, especially one for receiving a coin or other item.

A video slot game is a type of gambling machine that accepts cash or paper tickets with barcodes, and gives the player a chance to win credits based on the pattern of symbols displayed on the reels. Many slots have a specific theme, with symbols and bonus features aligned with that theme. Players activate the machine by pressing a lever or button (either physical or virtual), which causes the reels to spin and then stop at various positions to rearrange the symbols. When the symbols match a winning combination, the player earns credits according to the paytable.

In the past, all slot machines used revolving mechanical reels to display and determine results. The original machines had three physical reels with 10 symbols on each, for a total of 103 = 1,000 possible combinations. When manufacturers incorporated microprocessors into their machines, they were able to assign different probability weighting to each symbol on each reel, allowing them to create larger jackpots by making losing symbols appear more often than winning ones.

Modern slot machines use random number generators to pick the sequence of symbols stopped in each spin, ensuring that every spin has an equal chance of winning. However, some players have figured out ways to manipulate the odds of a particular machine by timing their actions or studying the patterns of the reels. Psychologists have found that people who play video slots reach a debilitating level of addiction to gambling at three times the rate of those who gamble in person.