Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

tailstock
     n : 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