Flank (side), in fortification, is the part of a bastion which extends from the face to the curtain, commanding and defending the latter as far as the next bastion. The flank of an army is properly the right or left wing, but may be used of any of its divisions. Flank company is the company posted on the extreme right or left of a battalion. "To flank" a position is to guard it with field-works; the word is also used of detaching troops for the purpose of defending one's own flank or attacking that of the enemy. Flanking parties are cavalry or infantry thrown out to reconnoitre or to protect a column while marching.