The Collaborative International Dictionary of English v.0.48:

Tailstock \Tail"stock`\, n.
   The sliding block or support, in a lathe, which carries the
   dead spindle, or adjustable center. The headstock supports
   the live spindle.
   [1913 Webster]

WordNet (r) 3.0 (2006):

tailstock
    n 1: support consisting of the movable part of a lathe that
         slides along the bed in alignment with the headstock and is
         locked into position to support the free end of the
         workpiece