The Collaborative International Dictionary of English v.0.48:

Headstock \Head"stock`\ (-st[o^]k`), 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.
       [1913 Webster]

WordNet (r) 3.0 (2006):

headstock
    n 1: 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)