Reverse Trap

In the English anime community, a reverse trap is a girl that looks like a boy, typically in the sense of a character that's canonically a girl, but whose design leads the audience or other characters to mistake her for a boy. Whether a character counts as a reverse trap may vary from person to person.

A "trap" is an anime slang for a boy that looks like a girl. The "reverse" comes from the Japanese word gyaku 逆, which is prefixed to things in Japanese to reverse the genders, e.g. gyaku-haaremu 逆ハーレム, "reverse harem."

いっそアンタもモデル目指してみたら俺が？需要あると思うよつっても俺ファションとか知らんし

Manga: Centaur no Nayami, セントールの悩み (Chapter 3)

In Japanese
Tropes

In Japanese

To say "reverse trap" in Japanese, the closest anime slang would be:

onna no ko
雄んなの子
"Girl," onnanoko 女の子, written with "male," osu 雄.
- A wordplay based on otokonoko 男の娘, which is the closest Japanese equivalent for "trap," and which is a wordplay on "boy," otokonoko 男の子, written with "girl," musume 娘.

However, this term isn't very used. Instead, a more common, broader term would be:

booisshu
ボーイッシュ
"Boyish." Refer to girls that wear clothes, hairstyles, etc., that make them look boyish.

Although the "reverse" comes from gyaku 逆, given that many Japanese slangs that reverse genders have gyaku in it, and they're all translated to English as "reverse," there's no such thing as gyaku-trap in Japanese.

It's worth noting that "trap" doesn't match otokonoko perfectly, and similarly "reverse trap" won't match onnanoko either. A character that only looks the opposite gender when they're crossdressing aren't called such terms, even though they may get called trap, reverse trap in English.

dansou
男装
Crossdressing by wearing "male clothes."
dansou joshi
男装女子
Crossdressing girl.

Tropes

Drama

Compared to trap characters, reverse traps tend to have some pretty heavy, serious reason to be mistaken for a boy, to the point that, in some cases, the fact that a character is actually female is a huge spoiler in a series.

Often, you'll have a scenario in which women aren't respected for one reason or another, or aren't allowed to do certain things, so the girl has to pretend to be a boy, or was raised like a boy, or something like that.

Appearance

Reverse trap characters tend to feature shorter hair and flatter chests than you average female character, hence why they're mistaken for boys so much.

If the character is pretending to be a boy, she may wear a sarashi 晒 around her chest to make it appear flatter.

Konoe Subaru 近衛スバル, wearing a sarashi 晒 on her chest to hide the fact she's a girl.

Character: Konoe Subaru 近衛スバル
Anime: Mayo Chiki! まよチキ！ (Episode 2)

Common sense tells us that if a character is wearing a skirt, or a female student uniform, they're a girl, therefore, there should be no reverse traps wearing obviously female clothes. Unfortunately, however, common sense doesn't apply to anime.

Since there are so many traps and crossdressers in anime, what looks like a short-haired girl wearing a skirt may as well be a boy wearing a skirt, so any slightly androgynous character is in a quantum state of being both girl and boy until more information is available.

Pronouns

Some reverse traps will use masculine first person pronouns, like boku 僕 and ore 俺. Girls that use these pronouns are called bokukko ボクっ娘 and orekko オレっ娘 respectively.