Invisigender is a xenogender that feels, for whatever reason, partially or fully invisible. It can be used as a prefix for other genders, such as invisigirl, invisiboy, invisinonbinary, etc. It may also mean that ones gender identity is complex and cannot be understood by other indivduals.
A considered similar term is tenerbrigender, but the difference lies in that those who are tenerbrigender may feel features of their gender but cannot make out the specification, whilst those who are invisigender can make out their gender but feel it isn't visible or is invisible.
History
Invisigender was coined by Tumblr user Gendermark (now deactivated) on or before July 14 2014[1].
Flag
The flag was created on September 27, 2015 by Pride-Flags on Deviantart.