Source : Webster's Revised Unabridged Dictionary (1913)

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}.

   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.

                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}.

   3. (Arch.) The side of any building. --Brands.

   4. That part of the acting surface of a gear wheel tooth that
      lies within the pitch line.

   {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.