Source : Webster's Revised Unabridged Dictionary (1913)

Megaphone \Meg"a*phone\, n. [Mega- + Gr. ? 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.

Source : WordNet®

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