1. An assistant to or a substitute for someone else in a higher rank or an assistant with the power to act when his superior is absent.
2. An officer in the U.S. Navy or Coast Guard of a rank above lieutenant junior grade, or an officer in the British or Canadian navies of a rank above sub-lieutenant.
3. A U.S. police or fire department officer of a rank above sergeant.
4. A first or second lieutenant in the U.S. Army, Air Force, or Marine Corps; where the first lieutenant is in a rank above a second lieutenant and below a rank of captain.
5. Etymology: from an Old French compound made up of lieu, "place" and tenant, "holding".

The word in Old French and the borrowed Middle English word lieutenant, first recorded near the end of the 14th century, referred to a person who acted for another as a deputy. This usage has continued, for example, in our term "lieutenant governor", the deputy of the governor and the one who replaces the governor if it is necessary.

In military references, lieutenant appears by itself as well as in compounds; such as, "first lieutenant" and "second lieutenant"; however, the original notion of the word in military usage was that the officer it referred to ranked below the next one who was higher and could replace him, if it were necessary; so, a lieutenant in the U.S. Army could replace a captain.