A sperm whale is a large whale with a large head that has a section in it which contains oil.
sperm whale in British English
noun
a large toothed whale, Physeter catodon, having a square-shaped head and hunted for sperm oil, spermaceti, and ambergris: family Physeteridae
Also called: cachalot
Word origin
C19: short for spermacetiwhale
sperm whale in American English
any of a family (Physeteridae) of toothed whales; esp., a large species (Physeter catodon) inhabiting warm seas: a closed cavity in its roughly square head contains sperm oil