medusa
Concise Oxford English Dictionary © 2008 Oxford University Press:
medusa /mɪˈdjuːzə, -sə/
▶noun (pl. medusae /-ziː, -siː/ or medusas) Zoology the free-swimming sexual form of a jellyfish or other coelenterate, typically umbrella-shaped with stinging tentacles around the edge. Compare with polyp.
– derivatives
medusoid adjective & noun.
medusoid adjective & noun.
– origin C18: named by assoc. with Medusa, a gorgon in Greek mythology with snakes in her hair.
'medusa' also found in these Oxford entries:

