The Collaborative International Dictionary of English v.0.48:

mortarboard \mortarboard\, mortar board \mortar board\n.
   1. A small square board with a handle beneath, for holding
      mortar; a hawk; used by masons to hold or carry mortar.
      [1913 Webster + WordNet 1.5]

   2. An academic cap topped by broad, projecting, stiff and
      flat square top, with a tassel attached to the top and
      hanging down. It was once worn by students in some
      colleges, but is now worn usually only at graduation
      ceremonies.
      [1913 Webster + WordNet 1.5]

WordNet (r) 3.0 (2006):

mortarboard
    n 1: a square board with a handle underneath; used by masons to
         hold or carry mortar [syn: mortarboard, hawk]
    2: an academic cap with a flat square with a tassel on top