![1girl bad_id bad_pixiv_id breasts collarbone fire gloves hat highres medium_breasts meleph_(xenoblade) military military_hat military_uniform pauldrons reverse_trap shi_shi_kaku shoulder_armor simple_background solo uniform white_gloves xenoblade_(series) xenoblade_2 yellow_eyes rating:s score:6 post #2971715](/cdn_image/preview/7e/97/7e9749b7623d520470deea465283de80.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.