A flank is the side of either a horse or a military unit.
The parts of a military unit are regularly called the "left flank", "right flank" and "center". The use of these terms came from a time when armies would clash in an open field, but is still used today. Throughout military history, a common tactic has been to try to catch the enemy forces in the flank, see below.
To flank is a verb, meaning to catch someone or something in the side (the flank, see above) or even from behind in order to attack them. The use of attack from an unexpected direction is generally very effective, and severely limits the offensive response possible for the attacked party. Flanking maneuvers (that is, movement in order to flank an enemy) are used on all levels of military tactics, from individual soldier up to army divisions.
See also