Someone who is neither/not wholly of the
binary genders; perhaps somewhere in the middle, entirely outside, partly one or the other, both simulataneously,
fluctuating, etc.
These all have proper names but non-binary is an
umbrella term for these things.
"The two
binary genders are
boy and girl, NOT male and female because those are sexes, so non-binary people are not
wholly one gender of the other."