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Longman Dictionary of Contemporary English
second lieutenant
noun
EXAMPLES FROM CORPUS
▪ During the brief period I spoke of, we had a second lieutenant take over the platoon.
▪ I opened the door as a young second lieutenant walked by.
▪ M in the 1960s, Hargrove also earned a commission as a second lieutenant in the U. S. Army.
Wiktionary
second lieutenant

n. 1 The lowest rank of a commissioned army officer, below a lieutenant. 2 The lowest rank of a commissioned officer in the United States Army, United States Air Force, or United States Marine Corps, below a first lieutenant. The rank of second lieutenant is equivalent to the naval rank of ensign.

WordNet
second lieutenant

n. a commissioned officer in the army or air force or marine corps holding the lowest rank

Wikipedia
Second lieutenant

Second lieutenant (called under-lieutenant in some countries) is a junior commissioned officer military rank in many armed forces.