noun
beautiful girl
Refers to a young, attractive female, often used in anime, manga, and light novels. Can also describe real people, but carries a slightly stylized or idealizing tone.
彼女はまるでアニメから出てきたような美少女だ。
She is a beautiful girl who looks like she stepped out of an anime.
このゲームの主人公は美少女キャラクターばかり
The main characters in this game are all beautiful girl characters.
美少年 is the male equivalent, referring to a beautiful young boy.
Compound of 美 (beautiful) + 少女 (young girl). The term became widely popular through anime and manga culture.