What is the thick layer of fat under a whale's skin called?

The thick layer of fat under a whale's skin is called blubber. Blubber is a specialized layer of vascularized adipose tissue found beneath the skin of whales, dolphins, and other marine mammals.

It helps whales stay warm in cold ocean water because its lipid-rich structure slows heat loss. Unlike fur, which traps air, blubber remains effective under the pressure of deep dives. It also stores energy for species that migrate, fast, or feed in different parts of the ocean.

Blubber can contribute to buoyancy and helps streamline a whale's body. Its thickness and composition vary with species, age, season, health, and reproductive condition; some marine mammals may carry blubber equal to a substantial share of their body mass.

Blubber is not the same as ordinary cooking fat, nor is it called lard or tallow. Historically, commercial whaling targeted it because rendered whale blubber produced whale oil used in lamps, candles, soap, and other products.