Definition of a reverse trap
A traditional 'trap' is a boy disguised as a female. Note that this is not simply boys who look feminine, but characters who are pretending to be the opposite sex or who are presumed to be by the narrative of the medium. 

Example 'traditional' trap:

Maria from Maria✝Holic



&amp; Steins;Gate's Ruka Urushibara (as you mentioned earlier)

Not a trap: Ryuunosuke Akasaka (The Pet Girl of Sakurasou)



A 'reverse trap' is simply the opposite - A female character pretending to be a male one. This is why Charlotte falls under this category. (Until she is outed to be a female). Similar to above, characters who are masculine are not strictly reverse traps.

Example 'reverse' trap:

Haruhi Fujioka - Ouran Highschool Host Club



Not a reverse trap: Yū Kashima from Nozaki-Kun