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 note
    n 1: a musical note having the time value of an eighth of a
         whole note [syn: eighth note, quaver]