The fleshy or muscular part of the side of an animal, between the ribs and the hip. Hence,

The side of an army, or of any division of an army, as of a brigade, regiment or battalion. To attack an enemy in flank, is to attack them on the side.

In fortification, that part of a bastion which reaches from the curtain to the face, and defends the opposite face, the flank and the curtain; or it is a line drawn from the extremity of the face towards the inside of the work.

To attack the side or flank of an army or body of troops; or to place troops so as to command or attack the flank.

To post so as to overlook or command on the side; as, to flank a passage.

To secure or guard on the side; as flanked with rocks.

To border; to touch.

To be posted on the side.