reverse trap

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. "masculine clothes/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.