Pronunciation: (lOO-ten'unt; in Brit. use, except in the navy, lef-ten'unt),[key] —n. 1. Mil. a. See first lieutenant. b. See second lieutenant. 2. U.S. Navy.a commissioned officer ranking between lieutenant junior grade and lieutenant commander.
3. a person who holds an office, civil or military, in subordination to a superior for whom he or she acts: If he can't attend, he will send his lieutenant.