The Collaborative International Dictionary of English v.0.48:

Crosshead \Cross"head`\ (-h?d), n. (Mach.)
   A beam or bar across the head or end of a rod, etc., or a
   block attached to it and carrying a knuckle pin; as the solid
   crosspiece running between parallel slides, which receives
   motion from the piston of a steam engine and imparts it to
   the connecting rod, which is hinged to the crosshead.
   [1913 Webster]

WordNet (r) 3.0 (2006):

crosshead
    n 1: a heading of a subsection printed within the body of the
         text [syn: crossheading, crosshead]
    2: metal block that connects to a piston; it slides on parallel
       guides and moves a connecting rod back and forth