From The Collaborative International Dictionary of English v.0.48 :
Mediety \Me*di"e*ty\, n. [L. medietas.]
The middle part; half; moiety. [Obs.] --Sir T. Browne.
[1913 Webster] Medievalism
Medieval
From WordNet (r) 3.0 (2006) :
mediety
n 1: one of two (approximately) equal parts [syn: moiety,
mediety]