A slot machine or fruit machine or poker machine or simply slot is a casino gambling machine with three or more reels which spin when a button is pushed. Slot machines are also known as one-armed bandits because slot machines were originally operated by a lever on the side of the machine (the one arm) instead of a button on the front panel, and because of their ability to leave the gamer penniless. Many modern machines still have a legacy lever in addition to the button.

A slot machine is a gambling machine, or a gambling device. (Modern slot machines are often called "electronic gaming devices".) People around the world play slot machines, and they have lots of colloquial phrases they use to refer to these games:

• One armed bandits (a US idiom)
• Fruit machines (in the UK)
• Poker machines or "pokies" (in Australia)

But slot machines are an American invention, and the phrase itself is actually a shorter version of the original name for the games: "penny-in-the-slot machines". The games were (until recently) always activated by putting a coin or a token into a slot and pulling a lever.

Slot machines are a relatively recent American invention too. The original slot machine was invented by Charles Fey and was called the Liberty Bell. In fact, his family still maintain a slot machine museum called the Liberty Bell where interested hobbyists can see historical examples of slot machines that are close to 100 years old.

Slot Machines Are Popular
No form of casino gambling is more popular than slot machine gambling. In most modern casinos, 65% to 75% of the casino's action is generated from the one-armed bandits. For a casino game that's only about a century old, that's pretty impressive. Especially when you consider that originally slot machines were intended to be a distraction for the ladies while their husbands played real gambling games like blackjack, poker, craps, and roulette.

Will slots remain this popular? My guess is that for the foreseeable future, they will. A slot machine is basically a mechanical or computerized version of a lottery game, and lotteries have been perennially popular for centuries. Slot machines might look very different 100 years from now, but they'll almost certainly still exist.

Mechanical Slot Machines
Slot machines were originally completely mechanical. A system of mechanical reels, stops, pulleys, and levers ran the entire mechanism. A slot machine reel would typically have 10 to 12 symbols on it. And a machine would have 3 reels.

Slot Machine Odds
The odds of getting 3 symbols to match were easily calculated. You just multiply the odds of each symbol's appearance times the odds of its appearance on the other reels. So if you had 12 symbols on 3 reels, the odds of any given symbol matching would be 1/12 X 1/12 X 1/12, or 1 in 1728. It's easy to see how a casino or gambling hall could make a profit with a machine like this, since the odds are so easy to calculate. If a casino wanted to make a 10% profit on each and every bet long term, they'd merely need to pay out 1556 coins any time 3 symbols hit. This is called the casino's advantage.

Modern Slot Machines
Modern slot machines don't work like this though, so the information is almost trivial. But understanding the principle behind the mechanical machines is illustrative of what goes on inside of a modern video slot machine too. A modern slot machine determines each outcome via the use of a random number generator.

Random Number Generators
A random number generator is a computer program which constantly generates numbers. When the spin button or lever is pulled on a modern slot machine, the number that the program has generated at that instant is used to determine the outcome of the spinning reels. Symbols can now be programmed to come up with any probability.