A soldier who was appointed by an officer to be his confidential assistant.  The aide wrote and delivered orders and held a position of responsibility which required him to know troop positions and where officer quarters were located.  The aide-de-camp was an officer by virtue of his position and he took orders from his commander only.

n. An officer who receives and transmits the orders of the general.

Senior officers were assisted in the daily management of their command by a more junior officer, the aide-de-camp.

a personal assistant whose responsibility it is to coordinate the personal affairs of the general-grade officer to whom he is appointed.  The rank of the aide-de-camp is a function of the rank of the [..]

(n) an officer who acts as military assistant to a more senior officer