Source : Webster's Revised Unabridged Dictionary (1913)

Headstock \Head"stock`\, n. (Mach.)
   A part (usually separate from the bed or frame) for
   supporting some of the principal working parts of a machine;
   as:
   (a) The part of a lathe that holds the revolving spindle and
       its attachments; -- also called {poppet head}, the
       opposite corresponding part being called a tailstock.
   (b) The part of a planing machine that supports the cutter,
       etc.

Source : WordNet®

headstock
     n : the stationary support in a machine or power tool that
         supports and drives a revolving part (as a chuck or the
         spindle on a lathe)