middle-aged woman who looks very young for her age (as if by magic); beautiful witch (literal)
A media-coined term popularized by women's magazines; often used admiringly for women in their 40s or older who maintain youthful looks. The literal meaning 'beautiful witch' is secondary and mainly used in wordplay or fantasy contexts.
She's in her 40s, but she's called a 'bimajo' (a woman who looks magically young).
雑誌で美魔女特集をよく見かける。
I often see special features on 'bimajo' in magazines.
A modern coined term from 美 (beauty), 魔 (magic), and 女 (woman), popularized in the 2000s by Japanese women's magazines. The exact origin is uncertain, but it plays on the idea of a 'magical' preservation of youth.