The Collaborative International Dictionary of English v.0.48:

Antecedency \An`te*ced"en*cy\, n.
   The state or condition of being antecedent; priority.
   --Fothherby.
   [1913 Webster]

WordNet (r) 3.0 (2006):

antecedency
    n 1: preceding in time [syn: priority, antecedence,
         antecedency, anteriority, precedence, precedency]
         [ant: posteriority, subsequence, subsequentness]