A meme from 4chan, means a man that looks like a woman, hence a trap.
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 trap do not need the male tag unless all of the other characters are male.
The opposite is a reverse trap.

(Copied from Danbooru)

Updated by tousatu over 13 years ago