Wild Symbol
A wild symbol acts as a substitute. If you have two matching symbols and a wild on the same payline, the wild fills in as a third match, completing the combination. It works like a joker in a card game. Wilds typically substitute for all regular symbols but not for scatters or other special symbols.
Modern slots have invented dozens of wild variations. Expanding wilds stretch to cover an entire reel. Sticky wilds stay locked in place for multiple spins. Walking wilds move one position left or right each spin. Multiplier wilds apply a 2x, 3x, or higher multiplier to any win they're part of. Some of the biggest slot payouts in history have come from stacked multiplier wilds during bonus rounds.
The number and behaviour of wilds directly affect how a slot plays. Slots with generous wild mechanics tend to feel more forgiving during dry spells. If the paytable shows that wilds only appear on reels 2, 3, and 4, the slot is deliberately limiting their impact.