Someone who tries on the mask of a gender other than their own when online.
There are many reasons why someone would pretend to be a girl, or pretend to be a boy online such as:
Many unskilled gender tourists over egg the pudding, by taking on a name like sexysexyhotkittengirl and describing their vast bouncing breasts and skimpy clothing in overenthusiastic terms.
Some people are extraordinarily paranoid about this, especially when indulging in netsex, and devise trick questions which will catagorically prove the 'true' gender of the other. These questions tend to be as sophisticated as pre-school and frequently revolve around menstruation and pantyhose sizes.
Wrongly calling a gender tourist out, though, is guaranteed to piss people off.
Gender tourism is mostly harmless, and frequently amusing unless it involves nasty meatspace surprises.