The Collaborative International Dictionary of English v.0.48:

Fireball \Fire"ball`\, n.
   1. (Mil.) A ball filled with powder or other combustibles,
      intended to be thrown among enemies, and to injure by
      explosion; also, to set fire to their works and light them
      up, so that movements may be seen.
      [1913 Webster]

   2. A rare phenomenon often associated with or caused by
      lightning, resembling a luminous ball of fire passing
      rapidly through the air or along solid objects, then
      disappearing, and sometimes exploding. It seldom lasts
      more than a few seconds. Also called ball lightning,
      globe lightning, globular lightning, or kugelblitz.
      [Webster 1913 Suppl. +PJC]

   3. A large mass of fire caused by a large explosion, as of
      inflammable liquids or a nuclear device. The larger
      fireballs, as of nuclear explosions, rise seemingly intact
      into the air and may reach high altitudes while still
      glowing.
      [PJC]

WordNet (r) 3.0 (2006):

fireball
    n 1: an especially luminous meteor (sometimes exploding) [syn:
         bolide, fireball]
    2: a highly energetic and indefatigable person [syn:
       powerhouse, human dynamo, ball of fire, fireball]
    3: a ball of fire (such as the sun or a ball-shaped discharge of
       lightning)
    4: the luminous center of a nuclear explosion