From Pluralpedia, the collaborative plurality dictionary
demi-plural (n., adj.)
A flag with 6 stripes, going from purple to white to light green.
Applies tosystems
CoinerTaliesin (spectre system), Frost System, Rosethorn System

Demi-plural is the state of being stuck between plural and singlet and not fully relating to either. Demi-plural may be more common among median systems, but is not exclusive to them. The term can be described as a sort of purgatory.