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