noun
leader of a group of juvenile delinquents; gang leader
Refers to the boss of a school-based or neighborhood youth gang, often in a delinquent subculture context. The term carries a rough, old-school image and is common in manga and anime.
彼は中学のとき、番長として恐れられていた。
He was feared as the gang leader when he was in junior high school.
その漫画の主人公は、転校先の学校で番長に
The protagonist of that manga becomes the boss of the delinquents at his new school.