Longman Dictionary of Contemporary English
Wiktionary
n. 1 The second lowest rank of a commissioned officer in the United States Army, Air Force, or Marine Corps, ranking above a second lieutenant and below a captain. The rank of first lieutenant is equivalent to the naval rank of lieutenant junior grade. 2 An equivalent rank in other military forces.
WordNet
n. a commissioned officer in the army or air force or marines ranking above a 2nd lieutenant and below a captain
Wikipedia
First lieutenant is a commissioned officer military rank in many armed forces and, in some forces, an appointment.
The rank of lieutenant has different meanings in different military formations (see comparative military ranks), but the majority of cases it is common for it to be sub-divided into a senior (first lieutenant) and junior (second lieutenant) rank. In navies it may relate to a particular post rather than a rank.
Usage examples of "first lieutenant".
He was first lieutenant now, he was one of the officers who had acquiesced, if nothing more, in the displacement of Sawyer from command, and it would take a victory to invest him with any virtue at all in the eyes of his superiors.
It was to Bush's credit that he felt sorrow at the death of Roberts before his mind recorded the fact that he was now first lieutenant of a ship of the line.
The man was at least a couple of centimeters taller than she was, with broad shoulders and a face that actually had a sort of rough handsomeness, and he wore the insignia of a first lieutenant in the People’.
The man was at least a couple of centimeters taller than she was, with broad shoulders and a face that actually had a sort of rough handsomeness, and he wore the insignia of a first lieutenant in the People's Marines, which she assumed denoted the same rank for State Security ground forces, as well.
Ormerod said, to which his first lieutenant nodded emphatic agreement.
The tired and dirty first lieutenant was the last officer the cavalry company had left.