![1girl adjusting_clothes adjusting_gloves black_footwear black_gloves black_suit character_name commentary_request crossdressing fate/grand_order fate_(series) formal frown gloves hair_between_eyes hair_ribbon highres long_hair low_ponytail necktie red_eyes red_neckwear red_ribbon reverse_trap ribbon silver_hair simple_background solo suit tapioka_(oekakitapioka) tomoe_gozen_(fate/grand_order) white_background rating:s score:14 post #3432584](/cdn_image/preview/0e/0d/0e0d640372461d2d8b10439e636bd8b3.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.