Hutton, Charles A Mathematical and Philosphical Dictionary

 FLANK
FLANK

, in Fortification, is that part of the bastion which reaches from the curtain to the face; and it defends the curtain, with the opposite face and flank.

Oblique or Second Flank, or Flank of the Curtan, is that part of the curtain from whence the face of the opposite bastion can be seen, being contained between the lines rasant and sichant, or the greater and less lines of defence; or the part of the curtain between the Flank and the point where the fichant line of defence terminates.

Covered, Low, or Retired Flank, is the platform of the casemate, which lies hid in the bastion; and is otherwise called the Orillon.

Fichant Flank, is that from whence a cannon playing, fires directly on the face of the opposite bastion.

Rasant or Razant Flank, is the point from whence the line of defence begins, from the conjunction of which with the curtain, the shot only raseth the face of the next bastion, which happens when the face cannot be discovered but from the Flank alone.

Simple Flanks, are lines going from the angle of the shoulder to the curtain; the chief office of which is for the defence of the moat and place.

Flank

, is also a term of war, used by way of analogy for the side of a battalion, army, &c, in contradistinction to the front and rear. So, to attack the enemy in Flank, is to discover and fire upon them on one side.