![2girls :o ahoge arrow_(projectile) arrow_through_heart artist_name bangs bare_arms blue_eyes blue_hair bow bowtie breasts brown_eyes brown_hair chain cigarette cleavage commentary_request cosplay crossdressing dress eyebrows_visible_through_hair feet_out_of_frame formal gloves hat hat_ribbon heart jacket kawacy leg_tattoo lifted_by_another long_hair long_sleeves looking_at_viewer marilyn_monroe marilyn_monroe_(cosplay) mouth_hold multiple_girls open_mouth pants parody pipimi plaid plaid_bow plaid_neckwear pleated_dress poptepipic popuko reverse_trap ribbon short_twintails sleeveless sleeveless_dress smile smoke smoking smug standing suit tattoo the_seven-year_itch twintails white_dress white_gloves white_headwear white_jacket white_suit wind wind_lift wing_collar rating:s score:36 post #3139952](/cdn_image/preview/69/ba/69ba5aa41097d9e2abd303373029e035.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.