You checked in on time, made it through security, found the gate, queued to get on board, struggled with the overhead lockers and settled back into your seat. And then you hear the captain say “We’re waiting for a slot.” What is a slot, and why can’t you take off as soon as you are ready?

A slot is a position in a sequence or series. It is also a term used in computer technology to describe an area on a motherboard where a specific type of device can be added, such as an expansion card. A slot can also refer to a particular position in an activity or program, for example, a visitor may book a time slot on a website to visit a museum.

The first slot machine was created by New Yorkers Sittman and Pitt in 1891. Their contraption was a mechanical reel machine that paid out credits if certain symbols lined up. Charles Fey improved on this design by replacing the poker symbols with ones like diamonds, spades, horseshoes, hearts and liberty bells. These fruit-themed machines were a hit and gave rise to the name of slot machines.

Modern slot machines use microprocessors to assign a probability to each symbol on a spinning reel. This means that, to the player, a certain symbol may seem to be close to being lined up with another one – but in reality, it’s very unlikely. This is why a machine can look so random to a novice: because the probabilities of different combinations are constantly changing.

While the original slot machine offered a limited number of pay lines and symbols, developers now offer an array of themed games with varying features and payouts. Some slots are grouped together to form progressive jackpots, while others feature Wild symbols that can substitute for other icons and unlock bonus levels or features.

In aviation, a slot is the space in which a plane can fly. Central flow management systems have dramatically reduced the number of times this has happened, but when it does, there are huge savings in terms of delays and fuel burn.