Which organ of a sperm whale inspired the name 'spermaceti'?

The spermaceti organ in its head, a huge cavity of waxy oil that early whalers mistook for semen - hence the name, and hence the whale's. It is a substantial fraction of the animal's body length, and the mistake is preserved in the taxonomy of the largest predator on Earth.

What it is actually for is still argued. The leading account is acoustics: the sperm whale produces the loudest sound of any animal, a focused click used for echolocation and possibly to stun prey, and the organ appears to work as a sound-focusing lens with the oil's density changing as it cools. A second, older proposal is buoyancy control for dives beyond a kilometre - cooling the wax makes it denser and helps the animal sink.

The oil is why they were hunted almost to nothing. Spermaceti burned brighter and cleaner than anything else available and made superb machine lubricant, so it lit European and American cities and greased the industrial revolution's precision instruments - it was still used in transmission fluid into the 1970s. A species was reduced by perhaps two thirds to provide lamp oil, and the substance was replaced by kerosene, which came out of the ground.