What is the name for a whale's tail fin?

A whale’s tail fin is called a fluke, or, more commonly, the paired tail flukes.

Unlike a fish’s vertically oriented tail, a whale’s flukes lie horizontally. Powerful muscles in the tail stock move them up and down, driving the whale through the water. The flukes provide propulsion, while the front flippers mainly help with steering and stabilization.

The two lobes are separated by a central notch, and their shape and markings vary between species. Humpback whales are especially recognizable by the black-and-white patterns on the undersides of their flukes. Researchers use these patterns, scars, and edge shapes for photo-identification, much like fingerprints.

A fluke is not the same as a flipper or a dorsal fin. Flippers are the paired front limbs; the dorsal fin, when present, sits on the back. Some whales, including gray whales, lack a prominent dorsal fin.