A machine that spins reels and pays out credits based on combinations of symbols. The payouts are governed by a paytable and can be adjusted with the controls on the machine. Slots can be played for fun or real money.
A modern slot machine is a complex piece of engineering, mathematical knowledge and psychological deception. Its design has evolved to reflect the changing tastes and technology of its customers. 
In the 1920s, newer machines accepted quarters and even silver dollars instead of paper tickets. Some machines were designed to appear more like skill games by adding buttons that allowed players to attempt to stop a specific reel at the desired time. Others incorporated the concept of a jackpot, using windows that displayed a growing pile of coins to lure players.
By the 1980s, electronic breakthroughs had replaced the mechanical sensors and switches in older slot machines. Manufacturers programmed their new machines to weight particular symbols so that they appeared more often on a given reel than other symbols. This reduced the odds of losing and increased the chances of winning.
The Random Number Generator central to a modern slot machine’s function has been around for decades. The actual spinning of the reels may be less sophisticated than a modern video game, but the music, visual displays and Rachel, Monica and the rest of the gang add up to keep gamblers coming back for more.
A recent article in The Atlantic by Nir Eyal tries to compare the addictive potential of video games and dating apps to that of slot machines. He concludes that while tech addictions can be dangerous, they are not in the same league as gambling.