The Collaborative International Dictionary of English v.0.48:

Cobblestone \Cob"ble*stone`\, n.
   A large pebble; a rounded stone not too large to be handled;
   a small boulder; -- used for paving streets and for other
   purposes.
   [1913 Webster]

WordNet (r) 3.0 (2006):

cobblestone
    n 1: rectangular paving stone with curved top; once used to make
         roads [syn: cobble, cobblestone, sett]
    v 1: pave with cobblestones [syn: cobble, cobblestone]