The Collaborative International Dictionary of English v.0.48:

faithfulness \faithfulness\ n.
   the trait of being faithful.

   Syn: fidelity.
        [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

Faithful \Faith"ful\, a.
   1. Full of faith, or having faith; disposed to believe,
      especially in the declarations and promises of God.
      [1913 Webster]

            You are not faithful, sir.            --B. Jonson.
      [1913 Webster]

   2. Firm in adherence to promises, oaths, contracts, treaties,
      or other engagements.
      [1913 Webster]

            The faithful God, which keepeth covenant and mercy
            with them that love him.              --Deut. vii.
                                                  9.
      [1913 Webster]

   3. True and constant in affection or allegiance to a person
      to whom one is bound by a vow, by ties of love, gratitude,
      or honor, as to a husband, a prince, a friend; firm in the
      observance of duty; loyal; of true fidelity; as, a
      faithful husband or servant.
      [1913 Webster]

            So spake the seraph Abdiel, faithful found,
            Among the faithless, faithful only he. --Milton.
      [1913 Webster]

   4. Worthy of confidence and belief; conformable to truth ot
      fact; exact; accurate; as, a faithful narrative or
      representation.
      [1913 Webster]

            It is a faithful saying.              --2 Tim. ii.
                                                  11.
      [1913 Webster]

   The Faithful, the adherents of any system of religious
      belief; esp. used as an epithet of the followers of
      Mohammed.

   Syn: Trusty; honest; upright; sincere; veracious;
        trustworthy. -- Faith"ful*ly, adv. -Faith"ful*ness,
        n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

faithfulness
    n 1: the quality of being faithful [syn: fidelity,
         faithfulness] [ant: infidelity, unfaithfulness]