Pronunciation: (esh'u-lon"),[key] —n. 1. a level of command, authority, or rank: the top echelon of city officials. 2. a formation of troops, ships, airplanes, etc., in which groups of soldiers or individual vehicles or craft are arranged in parallel lines, each to the right or left of the one in front, so that the whole presents the appearance of steps.
3. one of the groups of a formation so arranged.
4. Also called ech'elon grat"ing.Spectroscopy. a diffraction grating that is used in the resolution of fine structure lines and consists of a series of plates of equal thickness stacked in staircase fashion.