A slot is a dynamic placeholder on a web page that waits for or calls out to content. It is not to be confused with a renderer, which specifies how that content will appear in the slot.

A player inserts cash or, in “ticket-in, ticket-out” machines, a paper ticket with a barcode into a slot and activates it by pressing a lever or button (either physical or virtual). The machine then spins the reels and stops them to rearrange symbols. If a winning combination is formed, the player receives credits based on the pay table. Symbols vary by machine but classics include fruits, bells and stylized lucky sevens. Most slots have a theme and bonus features aligned with it.

For decades, slot machines were considered frivolities and were confined to casino periphery. But then came the advent of video slots, which offered more complex rules and faster payouts. As a result, they attracted and befuddled an entirely new generation of players.

While slot machines aren’t as combustible as cigarettes or heroin, they do have the potential to cause gambling addiction. Nir Eyal’s 2014 book Hooked reveals how slot machines exploit people’s psychological vulnerabilities to make them addictive. Eyal interviews players and gamblers who have fallen victim to the enigma of slots. They all report experiencing a similar feeling of “being in the zone” that draws them back to their games time and again, regardless of whether they win or lose.