The Collaborative International Dictionary of English v.0.48:

Magistracy \Mag"is*tra*cy\, n.; pl. Magistracies. [From
   Magistrate.]
   1. The office or dignity of a magistrate. --Blackstone.
      [1913 Webster]

   2. The collective body of magistrates.
      [1913 Webster]

WordNet (r) 3.0 (2006):

magistracy
    n 1: the position of magistrate [syn: magistracy,
         magistrature]