The Collaborative International Dictionary of English v.0.48:

Grenade \Gre*nade"\, n. [F. grenade a pomegranate, a grenade, or
   Sp. granada; orig., filled with seeds. So called from the
   resemblance of its shape to a pomegranate. See Carnet,
   Grain a kernel, and cf. Pomegranate.] (Min.)
   A hollow ball or shell of iron filled with powder of other
   explosive, ignited by means of a fuse, and thrown from the
   hand among enemies.
   [1913 Webster]

   Hand grenade.
   (a) A small grenade of iron or glass, usually about two and a
       half inches in diameter, to be thrown from the hand into
       the head of a sap, trenches, covered way, or upon
       besiegers mounting a breach.
   (b) A portable fire extinguisher consisting of a glass bottle
       containing water and gas. It is thrown into the flames.
       Called also fire grenade.

   Rampart grenades, grenades of various sizes, which, when
      used, are rolled over the pararapet in a trough.
      [1913 Webster]

WordNet (r) 3.0 (2006):

grenade
    n 1: a small explosive bomb thrown by hand or fired from a
         missile

Moby Thesaurus II by Grady Ward, 1.0:

18 Moby Thesaurus words for "grenade":
   aerial bomb, antipersonnel bomb, bomb, bombshell, carcass,
   concussion grenade, depth bomb, depth charge, fire bomb,
   gas grenade, hand grenade, incendiary bomb, incendiary grenade,
   infernal machine, petard, tear-gas grenade, time bomb,
   wall grenade