A set of parts whose behaviour together is in none of them alone. A system is what the parts do, not what they are.
Greek σύστημα (systema, a whole compounded of several parts, an organised body), from syn (together) + histanai (to stand, to set up); what the parts do together, and not what any of them are alone, was the sense from the start.
A set of parts whose behaviour together is in none of them alone. What a system does, rather than what any single part of it is.