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Longman Dictionary of Contemporary English
staff officer
noun
EXAMPLES FROM CORPUS
▪ Richard doesn't have the manners to be a staff officer.
▪ Thereafter his war service was entirely as a staff officer.
Wiktionary
staff officer

n. (context military English) An officer that fulfills a non-combat support function, often in a headquarters or planning office.

WordNet
staff officer

n. a commissioned officer assigned to a military commander's staff