The Collaborative International Dictionary of English v.0.48:

Dimness \Dim"ness\, n. [AS. dimness.]
   1. The state or quality ? being dim; lack of brightness,
      clearness, or distinctness; dullness; obscurity.
      [1913 Webster]

   2. Dullness, or want of clearness, of vision or of
      intellectual perception. --Dr. H. More.

   Syn: Darkness; obscurity; gloom. See Darkness.
        [1913 Webster]

WordNet (r) 3.0 (2006):

dimness
    n 1: the state of being poorly illuminated [syn: dimness,
         duskiness]
    2: the property of lights or sounds that lack brilliance or are
       reduced in intensity [syn: dimness, subduedness]
    3: the quality of being dim or lacking contrast [syn: dimness,
       faintness]