The Collaborative International Dictionary of English v.0.48:

Inconstancy \In*con"stan*cy\, n. [L. inconstantia.]
   The quality or state of being inconstant; lack of constancy;
   mutability; fickleness; variableness.
   [1913 Webster]

         For unto knight there was no greater shame,
         Than lightness and inconstancie in love. --Spenser.
   [1913 Webster]

WordNet (r) 3.0 (2006):

inconstancy
    n 1: unfaithfulness by virtue of being unreliable or treacherous
         [syn: faithlessness, falseness, fickleness,
         inconstancy]
    2: the quality of being changeable and variable [syn:
       inconstancy, changefulness] [ant: constancy,
       stability]