Other forms: flanks; flanked; flanking
A flank is the side of something, whether of beef or a military formation.
Although flank is specifically used to talk about a cut of meat, or the right or left side of a military formation, we also use it in those same senses for other objects or even metaphorically. You might, for example, refer to the more conservative flank of a political party. This word can also be used as a verb. If you see someone walking down the street with bodyguards on either side, she's flanked by guards.