Whirlpool (うずしお, Whirl Tide) is a damage-dealing Water-type move introduced in the second generation. It was the first HM item based move to lose its HM status, and was later followed by Flash, Dive, Rock Smash, and then Rock Climb. In HeartGold and SoulSilver, it regained its HM status, though as HM05 rather than HM06.
Outside of battle
By leveling up