The Collaborative International Dictionary of English v.0.48:

Flank \Flank\ (fl[a^][ng]k), n. [F. flanc, prob. fr. L. flaccus
   flabby, with n inserted. Cf. Flaccid, Flanch, Flange.]
   1. The fleshy or muscular part of the side of an animal,
      between the ribs and the hip. See Illust. of Beef.
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   2. (Mil.)
      (a) The side of an army, or of any division of an army, as
          of a brigade, regiment, or battalion; the extreme
          right or left; as, to attack an enemy in flank is to
          attack him on the side.
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                When to right and left the front

                Divided, and to either flank retired. --Milton.
      (b) (Fort.) That part of a bastion which reaches from the
          curtain to the face, and defends the curtain, the
          flank and face of the opposite bastion; any part of a
          work defending another by a fire along the outside of
          its parapet. See Illust. of Bastion.
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   3. (Arch.) The side of any building. --Brands.
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   4. That part of the acting surface of a gear wheel tooth that
      lies within the pitch line.
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   Flank attack (Mil.), an attack upon the side of an army or
      body of troops, distinguished from one upon its front or
      rear.

   Flank company (Mil.), a certain number of troops drawn up
      on the right or left of a battalion; usually grenadiers,
      light infantry, or riflemen.

   Flank defense (Fort.), protection of a work against undue
      exposure to an enemy's direct fire, by means of the fire
      from other works, sweeping the ground in its front.

   Flank en potence (Mil.), any part of the right or left wing
      formed at a projecting angle with the line.

   Flank files, the first men on the right, and the last on
      the left, of a company, battalion, etc.

   Flank march, a march made parallel or obliquely to an
      enemy's position, in order to turn it or to attack him on
      the flank.

   Flank movement, a change of march by an army, or portion of
      one, in order to turn one or both wings of the enemy, or
      to take up a new position.

   Flanks of a frontier, salient points in a national
      boundary, strengthened to protect the frontier against
      hostile incursion.

   Flank patrol, detachments acting independently of the
      column of an army, but patrolling along its flanks, to
      secure it against surprise and to observe the movements of
      the enemy.
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