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Wiktionary
sergeant first class

n. (context US Army English) A non-commissioned officer in the United States Army, ranking above a staff sergeant and below a master sergeant or first sergeant.

WordNet
sergeant first class

n. a sergeant in the army above the rank of staff sergeant and below master sergeant [syn: first sergeant]

Wikipedia
Sergeant first class

Sergeant First Class is a military rank in some militaries and other uniformed organizations around the world, typically that of a senior non-commissioned officer.