The Collaborative International Dictionary
Letheon
Letheon \Le"the*on\ (l[=e]"th[-e]*[o^]n), n. [NL., fr. Gr. lh`qh.] (Med.) Sulphuric ether used as an an[ae]sthetic agent. [R.]
Wiktionary
letheon
n. (context medicine archaic English) sulphuric ether used as an anaesthetic agent.