Military term for officers who assist a commander in planning and operations. Not a common everyday word.
He serves as a general staff officer in the division.
参謀 is a more common term for a staff officer or advisor, while 一般幕僚 specifically refers to the general staff in a military context.
Compound of 一般 (general) and 幕僚 (staff officer). The exact historical derivation is uncertain; the term is used in modern military contexts.