lieu·ten·ant 
(l-tĕnənt)
(l-tĕnənt)n.
1.
a. A commissioned rank in the US Navy or Coast Guard that is above lieutenant junior grade and below lieutenant commander.
b. A first lieutenant.
c. A second lieutenant.
d. One who holds the rank of lieutenant, first lieutenant, or second lieutenant.
2. (lĕf-tĕnənt) A commissioned officer in the British and Canadian navies ranking just below a lieutenant commander.
3. An officer in a police or fire department ranking below a captain.
4. One who acts in place of or represents a superior; an assistant or deputy: the organized crime figure and his lieutenants.
[Middle English, deputy, from Old French : lieu, place; see LIEU + tenant, present participle of tenir, to hold (from Latin tenēre; see ten- in the Appendix of Indo-European roots).]
lieu·tenan·cy n.