Merrian-Webster online says:

Etymology: French échelon, literally, rung of a ladder,

1 a (1) : an arrangement of a body of troops with its units each somewhat to the left or right of the one in the rear like a series of steps (2) : a formation of units or individuals resembling such an echelon (3) : a flight formation in which each airplane flies at a certain elevation above or below and at a certain distance behind and to the right or left of the airplane ahead b : any of several military units in echelon formation; also : any unit or group acting in a disciplined or organized manner
2 a : one of a series of levels or grades in an organization or field of activity b : a group of individuals at a particular level or grade in an organization s of the bureaucracy