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).
Across English-speaking communities, these characters are referred to in a number of different ways with no single standard term. The most common are "trap" and "femboy", and less frequently "tomgirl" or "otokonoko" are used.
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 otoko no ko 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.
