The Collaborative International Dictionary of English v.0.48:

Constancy \Con"stan*cy\, n. [L. constantia: cf. F. constance.
   See Constant.]
   1. The state or quality of being constant or steadfast;
      freedom from change; stability; fixedness; immutability;
      as, the constancy of God in his nature and attributes.
      [1913 Webster]

   2. Fixedness or firmness of mind; persevering resolution;
      especially, firmness of mind under sufferings, steadiness
      in attachments, or perseverance in enterprise; stability;
      fidelity.
      [1913 Webster]

            A fellow of plain uncoined constancy. --Shak.
      [1913 Webster]

            Constancy and contempt of danger.     --Prescott.

   Syn: Fixedness; stability; firmness; steadiness; permanence;
        steadfastness; resolution. See Firmness.
        [1913 Webster]

WordNet (r) 3.0 (2006):

constancy
    n 1: the quality of being enduring and free from change or
         variation; "early mariners relied on the constancy of the
         trade winds" [syn: constancy, stability] [ant:
         changefulness, inconstancy]
    2: (psychology) the tendency for perceived objects to give rise
       to very similar perceptual experiences in spite of wide
       variations in the conditions of observation [syn:
       constancy, perceptual constancy]
    3: faithfulness and dependability in personal attachments
       (especially sexual fidelity)