The Sperm Whale was named after the milky-white semiliquid waxy substance "spermaceti" found in its head and originally mistaken for sperm. The name derives from the late Latin sperma ceti (sperma is actually a loan word from Greek) meaning 'sperm of the whale' (strictly, 'sperm of the sea monster'). The common name for the species is actually an apocopation of Spermaceti Whale.
The substance is not of course the whale's semen; it was mistaken for such by early whalers. Spermaceti is found in the spermaceti organ or case in front of and above the skull of the whale and also in the so-called junk which is right at the front of the whale's head just above the upper jaw. The case consists of a soft white substance saturated with spermaceti. The junk is a more solid substance.