Nan0gender (also spelled NaN0gender and nanogender) refers to someone who has a gender that makes up a very small part of their total gender, and is mostly something else. For example, a nan0boy would feel very slightly male but mostly something else. The "something else" can be any gender or combination of genders, including agender. Nan0gender may overlap with libragender or themisgender, but generally a nan0gender is smaller than those genders.
Nan0girl, as it was originally spelled, was first created by a transmedicalist to mock the identity demigender. It was reclaimed by Tumblr user Captaintightpanties and boosted by Tumblr users Genderpunkrock and Hyaenahart. The spelling "NaN0girl" comes from the reclaimer, Captaintightpanties. According to them "NaN means 'Not a Number', and is a common error when programming if you try to call something that isn't a number when the computer expects one". The alternate spellings are stylistic and have no influence on the meaning.