The Collaborative International Dictionary of English v.0.48:

Accidence \Ac"ci*dence\, n. [A corruption of Eng. accidents, pl.
   of accident. See Accident, 2.]
   1. The accidents, of inflections of words; the rudiments of
      grammar. --Milton.
      [1913 Webster]

   2. The rudiments of any subject. --Lowell.
      [1913 Webster]

WordNet (r) 3.0 (2006):

accidence
    n 1: the part of grammar that deals with the inflections of
         words [syn: inflectional morphology, accidence]