![2girls a5a5_ygo breasts closed_eyes collarbone fire gameplay_mechanics gloves hat kagutsuchi_(xenoblade) long_hair medium_breasts meleph_(xenoblade) military military_hat military_uniform multiple_girls nude pauldrons purple_hair reverse_trap short_hair shoulder_armor simple_background smile translation_request uniform xenoblade_(series) xenoblade_2 yuri rating:q score:4 post #3107093](/cdn_image/preview/dd/00/dd00a092da4134409efe7cc730815924.jpg)
Per trap, a female character who looks and may or may not act masculine. Most likely also dresses like a male, although unlike mere crossdressing the character is assumed by others to be male until demonstrated otherwise.
As a rule of thumb, if you can correctly tell the gender with complete certainty from the thumbnail alone, it's not a reverse trap.
Often indicated on Pixiv by the 男装 (Dansou lit. "Disguising oneself as a man") and 男装の麗人 (Dansou no Reijin lit. "A beautiful woman disguised as a man") tags. See also tomboy, which may overlap. pool #6118 may also overlap in some places, but is not necessarily the standard.