Demigender is an umbrella term for genders, in which one is partially, but not fully a given gender. It can also describe those who feel a slight connection to a gender, but not enough to fully identify as that gender, party identify as one gender and partly another/other genders. Some parts of their gender may or may not be known. 
Demigender individuals may identify as another gender in combination with this term, such as demigender and bigender. Demigender can also be an identity in of itself, which includes individuals who feel a partial connection to the concept of gender, rather than being connected to a specific gender/group of genders (see demiboy, demigirl and similar). Demigender individuals often identify as non-binary and may additionally identify as transgender, though not always. Demigender individuals can also be transmasculine, transfeminine, something else or neither.
Being demigender does not necessarily mean that one is partially a given gender and partially agender or genderless, though it can be. One can be, for example, a demiboy and a demigirl at the same time, meaning part of one's gender is boy and the other part is girl. The "other part" of one's gender can be any gender or combination of genders. In addition to that, any part of their gender can be static, fluid, fluctuating, fluix or something else entirely.
The opposite of demigender is hypergender.
Any gender can have a demigender version of it, created by adding the demi- prefix in front of the gender. Some examples of demigenders include:
- Demiboy: Partially a man or partially masculine.
- Demifluid: Partially fluid, with the other part being static.
- Demigirl: Partially a woman or partially feminine.
- Demineutrois: Partially neutrois.
- Deminonbinary: Partially non-binary.
- Demixenine: Partially xenine or xenogender.
- Demiagender: Partially agender, partially another gender (example: partially agender, partially aporagender).
- Demimaverique: Partially maverique.
- Demiandrogyne: Partially androgynous.
- Demiflux: Partially fluctuating in intensity. The other part might be static.
History and coining
It is unknown who coined the term originally, however it has been around since at least 2010. It is likely that the term was coined much earlier.
The prefix 'demi-' means 'half' or partial' and originates from Latin. It being at the beginning of the term implies the partial connection to gender(s) and/or the concept of it.
In 2021, an Instagram user known by the name of bluebolt712 made December 15th a Demigender Awareness and Pride day.