![1girl dark_skin gloves hybridmink long_hair mask red_eyes reverse_trap sheik solo super_smash_bros. surcoat the_legend_of_zelda the_legend_of_zelda:_breath_of_the_wild the_legend_of_zelda:_ocarina_of_time turban rating:s score:12 post #3306498](/cdn_image/preview/fb/0b/fb0b7331388d82053f46e802b0286ce0.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.