The Collaborative International Dictionary of English v.0.48:

Megaphone \Meg"a*phone\, n. [Mega- + Gr. fwnh` voice.]
   A device to magnify sound, or direct it in a given direction
   in a greater volume, as a very large funnel used as an ear
   trumpet or as a speaking trumpet.
   [1913 Webster]

WordNet (r) 3.0 (2006):

megaphone
    n 1: a cone-shaped acoustic device held to the mouth to
         intensify and direct the human voice