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

WordNet (r) 3.0 (2006):

mediety
    n 1: one of two (approximately) equal parts [syn: moiety,
         mediety]