The Collaborative International Dictionary of English v.0.48:

Diacritic \Di`a*crit"ic\, Diacritical \Di`a*crit"ic*al\, a. [Gr.
   ?, fr. ? to separate, distinguish; dia` through + ? to
   separate. See Critic.]
   That separates or distinguishes; -- applied to points or
   marks used to distinguish letters of similar form, or
   different sounds of the same letter, as, [=a], [a^], [aum],
   [=o], [o^], etc. "Diacritical points." --Sir W. Jones.
   [1913 Webster]

         A glance at this typography will reveal great
         difficulties, which diacritical marks necessarily throw
         in the way of both printer and writer.   --A. J. Ellis.
   [1913 Webster]

WordNet (r) 3.0 (2006):

diacritic
    adj 1: capable of distinguishing; "students having superior
           diacritic powers"; "the diacritic elements in culture"-
           S.F.Nadel [syn: diacritic, diacritical]
    n 1: a mark added to a letter to indicate a special
         pronunciation [syn: diacritical mark, diacritic]