The Collaborative International Dictionary of English v.0.48:

Sixty-fourth \Six`ty-fourth"\, a.
   Constituting or being one of sixty-four equal parts into
   which a thing is divided.
   [1913 Webster]

   Sixty-fourth note (Mus.), the sixty-fourth part of a whole
      note; a hemi-demi-semiquaver.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sixty-fourth note
    n 1: a musical note having the time value of a sixty-fourth of a
         whole note [syn: sixty-fourth note, hemidemisemiquaver]