The Collaborative International Dictionary of English v.0.48:

Eighth \Eighth\, n.
   1. The quotient of a unit divided by eight; one of eight
      equal parts; an eighth part.
      [1913 Webster]

   2. (Mus.) The interval of an octave.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Eighth \Eighth\ ([=a]tth), a. [AS. eahto[eth]a.]
   1. Next in order after the seventh.
      [1913 Webster]

   2. Consisting of one of eight equal divisions of a thing.
      [1913 Webster]

   Eighth note (Mus.), the eighth part of a whole note, or
      semibreve; a quaver.
      [1913 Webster]

WordNet (r) 3.0 (2006):

eighth
    adj 1: coming next after the seventh and just before the ninth
           in position [syn: eighth, 8th]
    n 1: position eight in a countable series of things
    2: one part in eight equal parts [syn: one-eighth, eighth]

Moby Thesaurus II by Grady Ward, 1.0:

27 Moby Thesaurus words for "eighth":
   contraoctave, double contraoctave, eightfold, four-foot octave,
   four-line octave, great octave, octadic, octagonal, octahedral,
   octal, octan, octaploid, octastyle, octave, octofid, octonary,
   octosyllabic, octuple, one-line octave, ottava, ottava alta,
   ottava bassa, small octave, subcontraoctave, tone row,
   two-foot octave, two-line octave