Is it ensure, insure, or assure?
1660s, "stopper, wedge," from Latin embolus "piston of a pump," from Greek embolos "peg, stopper; anything pointed so as to be easily thrust in," also "a tongue (of land), beak (of a ship)," from emballein (see emblem). Medical sense is from 1866. Related: Embolic.
embolus em·bo·lus (ěm'bə-ləs)
n. pl. em·bo·li (-lī')
A mass, such as an air bubble, a detached blood clot, or a foreign body, that travels in the bloodstream and lodges in a blood vessel, thus serving to obstruct or occlude such a vessel.
Plural emboli (ěm'bə-lī)