![1girl androgynous bad_id bad_pixiv_id bangs black_hair black_shirt blue_eyes closed_mouth collared_shirt dress_shirt grey_jacket hachiware hair_between_eyes highres idolmaster idolmaster_(classic) jacket kikuchi_makoto long_sleeves looking_afar reverse_trap shiny shiny_hair shirt short_hair simple_background solo upper_body white_background wing_collar rating:s score:8 post #3618253](/cdn_image/preview/8b/a6/8ba6f7dcb8472a09b9d74f943daa49d2.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.