Instructions given to be carried out. A command differs from a request by not leaving the answer open.
Latin mandare (to entrust, order), from manus (hand) + dare (to give) — literally to place a task into someone's hand; strengthened by the intensive com- into Old French comander, an order that leaves no room to refuse.
An instruction given to be carried out rather than proposed. What separates a command from a request is exactly this: nothing is left open for an answer.