Per trap, a female character who looks or acts male. 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 trap. Posts containing a reverse trap do not require the male tag either.

(Copied from Danbooru)

Updated by Aurelia about 15 years ago