A sticky wild is a wild that remains fixed in its position for more than one spin. While it stays, every following spin has that wild in place, which raises the chance of wins through that position.
How long it sticks depends on the game. Some sticky wilds stay for a set number of spins, some for a respin sequence, and many stay for the whole of a free spins round, building up as more land. A screen that fills with sticky wilds late in a feature is where many games' biggest results come from.
Sticky wilds are sometimes combined with multipliers or with expanding wilds. Their value is part of the published RTP. See our sticky wilds feature page.