Source : Webster's Revised Unabridged Dictionary (1913)

Instroke \In"stroke`\, n.
   An inward stroke; specif., in a steam or other engine, a
   stroke in which the piston is moving away from the crank
   shaft; -- opposed to {outstroke}.

Source : WordNet®

instroke
     n : the stroke of an engine piston moving away from the
         crankshaft