The Collaborative International Dictionary of English v.0.48:

Reliable \Re*li"a*ble\ (r?-l?"?-b'l), a.
   Suitable or fit to be relied on; worthy of dependance or
   reliance; trustworthy. "A reliable witness to the truth of
   the miracles." --A. Norton.
   [1913 Webster]

         The best means, and most reliable pledge, of a higher
         object.                                  --Coleridge.
   [1913 Webster]

         According to General Livingston's humorous account, his
         own village of Elizabethtown was not much more
         reliable, being peopled in those agitated times by
         "unknown, unrecommended strangers, guilty-looking
         Tories, and very knavish Whigs."         --W. Irving.
   [1913 Webster]

   Note: Some authors take exception to this word, maintaining
         that it is unnecessary, and irregular in formation. It
         is, however, sanctioned by the practice of many careful
         writers as a most convenient substitute for the phrase
         to be relied upon, and a useful synonym for
         trustworthy, which is by preference applied to persons,
         as reliable is to things, such as an account,
         statement, or the like. The objection that adjectives
         derived from neuter verbs do not admit of a passive
         sense is met by the citation of laughable, worthy of
         being laughed at, from the neuter verb to laugh;
         available, fit or able to be availed of, from the
         neuter verb to avail; dispensable, capable of being
         dispensed with, from the neuter verb to dispense. Other
         examples might be added.
         [1913 Webster] -- Re*li"a*ble*ness, n. --
         Re*li"a*bly, adv.
         [1913 Webster]

WordNet (r) 3.0 (2006):

reliableness
    n 1: the quality of being dependable or reliable [syn:
         dependability, dependableness, reliability,
         reliableness] [ant: undependability,
         undependableness, unreliability, unreliableness]