A male character who looks female. Also known as "femboy", or less frequently as "tomgirl".
The Japanese term 男の娘 (otoko no ko) is a pun on 男の子 (otoko no ko), meaning "boy", replacing 子 with the ko reading of 娘 (commonly musume, girl).
Some people use "femboy" to describe anyone who appears or acts feminine, even if they don't convincingly pass as female. If you can correctly tell the character is male with complete certainty from the thumbnail alone without looking at the genitalia, it's not this.
While many traps engage in crossdressing, it is not a requirement. Sufficiently feminine characters can pass as girls even when near-nude or dressed in typically masculine clothing.
The English term for trap originated in 4chan circa 2004, derived from Admiral Ackbar's quote "It's a trap!". For the literal definition of the word "trap", see trapped or booby trap.
