The Collaborative International Dictionary of English v.0.48:

Distinctness \Dis*tinct"ness\, n.
   1. The quality or state of being distinct; a separation or
      difference that prevents confusion of parts or things.
      [1913 Webster]

            The soul's . . . distinctness from the body.
                                                  --Cudworth.
      [1913 Webster]

   2. Nice discrimination; hence, clearness; precision; as, he
      stated his arguments with great distinctness.

   Syn: Plainness; clearness; precision; perspicuity.
        [1913 Webster]

WordNet (r) 3.0 (2006):

distinctness
    n 1: the quality of being sharp and clear [syn: distinctness,
         sharpness] [ant: blurriness, fogginess, fuzziness,
         indistinctness, softness]
    2: the state of being several and distinct [syn: discreteness,
       distinctness, separateness, severalty]
    3: the quality of being not alike; being distinct or different
       from that otherwise experienced or known [syn: otherness,
       distinctness, separateness]