The Collaborative International Dictionary of English v.0.48:

Lightning \Light"ning\ (l[imac]t"n[i^]ng), n. [For lightening,
   fr. lighten to flash.]
   1. A discharge of atmospheric electricity, accompanied by a
      vivid flash of light, commonly from one cloud to another,
      sometimes from a cloud to the earth. The sound produced by
      the electricity in passing rapidly through the atmosphere
      constitutes thunder.
      [1913 Webster]

   2. The act of making bright, or the state of being made
      bright; enlightenment; brightening, as of the mental
      powers. [R.]
      [1913 Webster]

   Ball lightning, a rare form of lightning sometimes seen as
      a globe of fire moving from the clouds to the earth.

   Chain lightning, lightning in angular, zigzag, or forked
      flashes.

   Heat lightning, more or less vivid and extensive flashes of
      electric light, without thunder, seen near the horizon,
      esp. at the close of a hot day.

   Lightning arrester (Telegraphy), a device, at the place
      where a wire enters a building, for preventing injury by
      lightning to an operator or instrument. It consists of a
      short circuit to the ground interrupted by a thin
      nonconductor over which lightning jumps. Called also
      lightning discharger.

   Lightning bug (Zool.), a luminous beetle. See Firefly.

   Lightning conductor, a lightning rod.

   Lightning glance, a quick, penetrating glance of a
      brilliant eye.

   Lightning rod, a metallic rod set up on a building, or on
      the mast of a vessel, and connected with the earth or
      water below, for the purpose of protecting the building or
      vessel from lightning.

   Sheet lightning, a diffused glow of electric light flashing
      out from the clouds, and illumining their outlines. The
      appearance is sometimes due to the reflection of light
      from distant flashes of lightning by the nearer clouds.
      [1913 Webster]

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]