Existigender is a gender that only exists when one think actively thinks about it. When one isn't actively thinking about and exploring their gender/gender identity, they do not have one, or it feels quiet until called to attention. One feels genderless until they think about their gender, at which point they can "choose" which gender to have, so they're genderless until they actively think about their gender.
Subsets
Existigender can be combined with any other gender to specify the nature of ones existigender.
Existiboy
An existiboy is someone who is only a boy when thinking about/exploring their gender/gender identity. When not doing so, their gender is simply not there.
Existigirl
An existigirl is someone who is only a girl when thinking about/exploring their gender/gender identity. When not doing so, their gender is simply not there.
Existinonbinary
An existinonbinary person is someone who is only non-binary when thinking about/exploring their gender/gender identity. When not doing so, there gender is simply not there.
History
Existigender was coined some time in 2013 or 2014. The exact date or coiner is unknown, but it was on the mogai archive on tumblr. The creator of the existigender flag is also unknown.