An expanding wild is a wild symbol that grows to cover more positions after it lands, usually the whole reel and sometimes a shape defined by the game. Like any wild, it substitutes for regular symbols (usually not scatters) to complete winning combinations, so turning one position into three or four can create several wins at once across different paylines.
Games handle the expansion in different ways. In some, every wild expands whenever it appears; in others, expansion is random or limited to free spins and bonus rounds. Many titles combine it with other mechanics, for example keeping an expanded reel in place while the other reels spin again. Because players have no control over when it triggers, the feature works as a pure bonus that can lift a modest spin into a much bigger win.
Tag games by mechanic
Players often look for features they already enjoy. Tagging games by mechanic, such as expanding wilds, cascades or hold-and-win, makes similar titles easier to find in the lobby.