The Collaborative International Dictionary of English v.0.48:

Apotome \A*pot"o*me\ ([.a]*p[o^]t"[-o]*m[-e]), n. [Gr. 'apotomh`
   a cutting off, fr. 'apote`mnein to cut off; 'apo` from +
   te`mnein to cut.]
   1. (Math.) The difference between two quantities
      commensurable only in power, as between [root]2 and 1, or
      between the diagonal and side of a square.
      [1913 Webster]

   2. (Mus) The remaining part of a whole tone after a smaller
      semitone has been deducted from it; a major semitone.
      [Obs.]
      [1913 Webster]