UFO is a booster in Candy Crush Saga that is used in all levels. It is unlocked at level 21. Each usage costs 20 gold bars. Using this booster will summon a UFO that puts 3 wrapped candies on the board (typically near blockers, jellies, or under ingredients) and sets them off.
Unlike the special candy version, this UFO will arrive from outside the screen, and will take action immediately.
On web versions, it was released when the episode Lullaby Lake was released.
When Milky Marina was released, players on web version received 3 free copies of this booster, and one more copy which could be used immediately upon clicking the "Show me!" button.
On mobile since v1.71, the UFO image on the button became bigger than in previous versions.