A slot is a dynamic placeholder that waits for content or actively calls out for content. A slot can be filled with content from a repository or with a targeter. Slots work in tandem with renderers to deliver content to the page.

The first electronic slots were electromechanical machines that used sensors to detect and count stops on a rotating drum or “reel”. In the 1960s, Bally Manufacturing introduced a new design that let players insert multiple coins and also allowed for diagonal payouts and multiplier jackpots. The new rig used integrated circuits and logic cards to replace the mechanical switches and wires.

Modern slots also use microprocessors to assign a different probability to each symbol on each reel. Consequently, a winning symbol might appear to hit on the payline very frequently, whereas it is much less likely to be displayed on the second or third reel. This can give the impression that a machine is so close to a jackpot that it must soon pay out.

Nir Eyal, a Stanford University professor and author of Hooked: The Science of Addictive Technology, was asked what distinguishes mobile games or dating apps from casino slots. He gave a range of answers that sounded at once comprehensive and defensive — that tech addiction never really plummets to the level of gambling addiction; that people prone to addiction will become addicted no matter what — before finally conceding that, in the end, all digital products function like slots.