![2girls ahoge amanda_o'neill black_coat black_vest blue_neckwear blue_ribbon blue_sash blue_skirt blue_vest blush bow closed_mouth coat collared_shirt commentary_request crossdressing formal green_background green_eyes hair_bow hand_on_hip hands_up hanna_england highres little_witch_academia long_sleeves luna_nova_school_uniform multicolored_hair multiple_girls neck_ribbon nose_blush one_eye_closed open_mouth orange_hair pants ponytail purple_pants red_hair reverse_trap ribbon sash school_uniform shirt short_eyebrows simple_background skirt standing suit tama_(tama-s) vest white_shirt wing_collar yellow_bow yellow_eyes rating:s score:13 post #2998503](/cdn_image/preview/57/0b/570b28434faff3c163fa18f3e9c30dfa.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.