男の娘 (otoko no ko) is a Japanese slang term used to refer to persons of male biological sex who have taken on the appearance and/or identity 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 wear girl's clothing or not. If you can correctly tell the gender with complete certainty from the thumbnail alone without looking at the genitalia, it's not this.
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.
On Pixiv the tag 男体化 is sometimes used for genderswap or otoko no ko art, but its meaning- "male's body", has multiple definitions. It can be a crossdresser who has a more masculine body (but isn't obviously a man), a woman who is very muscular, or it is sometimes used in place of otoko no ko (i.e. "he may have hips, but that's a boy's body").
- bishounen for boys with both masculine and feminine beauty traits
- reverse trap for ladies in masculine dress
- girly boy for boys that are feminine in personality, but not necessarily appearance
- newhalf for male genitalia, but otherwise female body
- cuntboy for female genitalia, but otherwise male body
- futanari for those with both male and female genitals
- Pixivision: A man can't be this cute! Traps