男の娘 (Otoko no ko) is a Japanese slang term used to refer to natal males (male at birth) with the appearance of the female gender. The term is a pun on 男の子 (otoko no ko), meaning "boy", replacing 子 with the ko reading of 娘 (commonly musume, girl).
Some communities have mistakenly taken to using the term as a word for feminine boys no matter if they convincingly look like girls or not. If you can correctly tell the character is male with complete certainty from the thumbnail alone without looking at the genitalia, it's not this.
While many otoko no ko engage in crossdressing, it is not a requirement. Sufficiently feminine characters can pass as girls even when nude or dressed in typically masculine clothing.
English-speaking fandom has traditionally used the word "trap" for this concept. Due to the negative implications of the term and its history (which 男の娘 largely does not share) as well as the term's ambiguity in English, its use is inappropriate for tagging purposes and it has been aliased.
