A made thing wielded by a hand toward an end — the tool taken up for a sitting rather than the machinery that runs on its own. An instrument is answerable to whoever holds it.
Latin instrumentum (a tool, implement, equipment; also a legal document), from instruere (to arrange, furnish, equip, teach) — in- (upon) + struere (to pile, build). The same root gives STRUCTURE and CONSTRUCT. The musical sense arrives in the 14th century and the legal one is older than both: an instrument was first anything that EQUIPS an action, whether a plough, a contract, or a fiddle.
A made thing wielded by a hand toward an end — the tool taken up for a sitting rather than the machinery that runs on its own. The Latin's breadth is the point: an instrument equips an act, and is answerable to whoever holds it. A system runs; an instrument is picked up.