The Collaborative International Dictionary of English v.0.48:

jackhammer \jackhammer\ n.
   a hammer having a strong steel cutting blade, driven by
   compressed air in multiple rapid strokes, and used for
   cutting through pavement, concrete, or other hard substances.

   Syn: air hammer, pneumatic hammer.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

jackhammer
    n 1: a hammer driven by compressed air [syn: air hammer,
         jackhammer, pneumatic hammer]