Demigenderflux is a combination of demigender, and genderflux. It's a term for when one has some connection to a given gender, but one's gender also fluctuates. For example, at their most neutral, they feel like they are nonbinary. At other times they may experience other genders, for example at their most feminine they feel like a demigirl, and at their most masculine they feel like a demiboy.
Demigenderflux can be used as an umbrella term for terms such as demiboyflux and demigirlflux, or it can be interpreted as the non-binary equivalent of those terms.