What is the hammer-shaped head of a hammerhead shark called?

The hammer-shaped head of a hammerhead shark is called a cephalofoil.

A cephalofoil is the flattened, laterally expanded structure that gives hammerheads their distinctive T-shaped or hammer-like profile. The family Sphyrnidae includes the familiar hammerhead sharks in the genus Sphyrna, along with the winghead shark in the genus Eusphyra. The shape varies among species: great hammerheads have a more pronounced T-shaped head, while bonnetheads have a rounded outline.

The cephalofoil is more than a striking silhouette. Its wide placement of the eyes provides an unusually broad field of view, and its expanded surface carries many electroreceptors called ampullae of Lorenzini. These sensory organs help sharks detect weak electrical signals produced by prey.

Older explanations often claimed that the head mainly functioned as a turning rudder. Research suggests the vertebral column contributes strongly to hammerhead manoeuvring, while the cephalofoil also supports sensory reception, visual depth perception, and possibly prey manipulation. “Rostrum” is a general term for a snout-like projection, not the hammerhead’s specialized head structure.