Source : Webster's Revised Unabridged Dictionary (1913)

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.

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

   {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} (Zo["o]l.), 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.

Source : WordNet®

heat lightning
     n : bright flashes of light near the horizon without thunder
         (especially on hot evenings); usually attributed to
         distant lightning that is reflected by clouds