The Collaborative International Dictionary of English v.0.48:

Echelon \Ech"e*lon\ ([e^]sh"e*l[o^]n), n. [F., fr. ['e]chelle
   ladder, fr. L. scala.]
   1. (Mil.) An arrangement of a body of troops when its
      divisions are drawn up in parallel lines each to the right
      or the left of the one in advance of it, like the steps of
      a ladder in position for climbing. Also used adjectively;
      as, echelon distance. --Upton (Tactics).
      [1913 Webster]

   2. (Naval) An arrangement of a fleet in a wedge or V form.
      --Encyc. Dict.
      [1913 Webster]

   Echelon lens (Optics), a large lens constructed in several
      parts or layers, extending in a succession of annular
      rings beyond the central lens; -- used in lighthouses.
      [1913 Webster]