What structure is present in dolphin but absent or greatly reduced in other whales?

The term "whale" is a word used to describe several different types of marine mammals, which include whales, dolphins and porpoises.

Together these three distinct animal groups make up what is known as the cetacean species.

While the name whale can sometimes refer to all cetaceans (whales, dolphins & porpoises) it usually excludes dolphins and porpoises in order to ensure that the three animal groups are easily identifiable.