![1girl androgynous arm_up belt belt_buckle belt_pouch brown_belt buckle coat covered_mouth cowboy_shot cropped expressionless fur_hat goggles goggles_on_headwear hair_between_eyes hand_on_wall hat highres holster jacket keyhole kino kino_no_tabi kuroboshi_kouhaku leather_belt long_sleeves looking_at_viewer official_art open_clothes open_coat pants pouch purple_eyes purple_hair purple_headwear purple_jacket purple_pants reverse_trap scan short_hair solo standing tomboy yellow_coat rating:s score:11 post #3504605](/cdn_image/preview/ad/af/adaf2eec79eed4c70f66d8efdfa7c7ef.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.