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

Moby Thesaurus II by Grady Ward, 1.0:

84 Moby Thesaurus words for "fireball":
   Jupiter Fulgur, Thor, aerolite, alcohol, ball lightning, benzine,
   bolide, bolt, bolt of lightning, briquette, burnable, butane,
   carbon, chain lightning, charcoal, chondrite, coal, coke,
   combustible, cosmic dust, dark lightning, dope, ethane, ethanol,
   firebolt, firing, flammable, flammable material, flying flame,
   forked lightning, fuel, fuel additive, fuel dope, fulguration,
   fulmination, gas, gas carbon, gasoline, heptane, hexane,
   inflammable, inflammable material, isooctane, jet fuel, kerosene,
   levin bolt, lightning, meteor, meteor crater, meteor dust,
   meteor shower, meteor swarm, meteor trail, meteor train,
   meteoric shower, meteorite, meteoroid, meteorolite, methane,
   methanol, micrometeorite, micrometeoroid, natural gas,
   oak-cleaving thunderbolts, octane, oil, paraffin, peat, pentane,
   propane, propellant, radiant, radiant point, rocket fuel,
   sheet lightning, shooting star, siderite, siderolite,
   stroke of lightning, tektite, thunderball, thunderbolt,
   thunderstroke, turf