In anime, a cyclops girl is an one-eyed monster girl, often with one impossibly large eye in the middle of her head.
The term for cyclops girl in Japanese is tangan-musume 単眼娘, "one-eyed girl," or tangan-shoujo 単眼少女, same meaning. They're also called "monoeye," monoai モノアイ, and hitotsume 一つ目, "one-eyed."
See the article about tangan 単眼 for details.
Anime: Star☆Twinkle Precure, スター☆トゥインクルプリキュア (Episode 16)