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.
That girl grew up into a beautiful young woman.
She is a beautiful girl who looks like she stepped out of an anime.
The main characters in this game are all beautiful girl characters.
The protagonist is depicted as a beautiful girl.
美人 is a general, everyday word for a beautiful woman, whereas 美少女 is narrower, referring only to young girls and often used in anime/manga contexts.
美少年 is the male counterpart of 美少女, referring to a beautiful young boy rather than a beautiful young girl.
美少年 and 美少女 look and sound similar but 美少年 means a beautiful boy while 美少女 means a beautiful girl.
美少女 is counted with 人 (or the more polite 名), as it refers to a person.
Compound of 美 (beautiful) + 少女 (young girl). The term became widely popular through anime and manga culture.