![2girls animal_ears bangs blunt_bangs bodysuit cat_ears eyebrows facial_mark gloves hat hood meleph_(xenoblade) military military_hat military_uniform multiple_girls niyah pauldrons reverse_trap ribbon short_hair shoulder_armor silver_hair simple_background smile t0gebakudan uniform white_gloves xenoblade_(series) xenoblade_2 yellow_bodysuit yellow_eyes rating:s score:7 post #2984757](/cdn_image/preview/db/87/db872892c52f9f5bc32b4ebd01be7eb5.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.