![3girls absurdres aqua_fire backless_outfit bare_back blonde_hair blue_eyes blush breasts bright_pupils broad_shoulders closed_eyes collarbone commentary coughing double-breasted dress facing_viewer fiery_hair fire frogbians girl_sandwich gloves gold_trim hand_to_own_mouth hat heart heart_hands heart_hands_duo highres kagutsuchi_(xenoblade) long_hair long_sleeves looking_at_viewer looking_away looking_to_the_side medium_breasts meleph_(xenoblade) military military_hat military_uniform multiple_girls nervous outside_border pauldrons purple_hair reverse_trap ribbed_gloves rinne_(xenoblade) sandwiched short_hair shoulder_armor sideways_glance silver_trim simple_background smile symbol_commentary tiara twintails uniform white_gloves white_pupils xenoblade_(series) xenoblade_2 yellow_eyes yuri rating:s score:8 post #3025605](/cdn_image/preview/80/d8/80d873791709abc53e2334294e4f06f4.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.