Source : Webster's Revised Unabridged Dictionary (1913)

Spontaneous \Spon*ta"ne*ous\, a. [L. spontaneus, fr. sponte of
   free will, voluntarily.]
   1. Proceding from natural feeling, temperament, or
      disposition, or from a native internal proneness,
      readiness, or tendency, without constraint; as, a
      spontaneous gift or proportion.

   2. Proceeding from, or acting by, internal impulse, energy,
      or natural law, without external force; as, spontaneous
      motion; spontaneous growth.

   3. Produced without being planted, or without human labor;
      as, a spontaneous growth of wood.

   {Spontaneous combustion}, combustion produced in a substance
      by the evolution of heat through the chemical action of
      its own elements; as, the spontaneous combustion of waste
      matter saturated with oil.

   {Spontaneous generation}. (Biol.) See under {Generation}.

   Syn: Voluntary; uncompelled; willing.

   Usage: {Spontaneous}, {Voluntary}. What is voluntary is the
          result of a volition, or act of choice; it therefore
          implies some degree of consideration, and may be the
          result of mere reason without excited feeling. What is
          spontaneous springs wholly from feeling, or a sudden
          impulse which admits of no reflection; as, a
          spontaneous burst of applause. Hence, the term is also
          applied to things inanimate when they are produced
          without the determinate purpose or care of man.
          ``Abstinence which is but voluntary fasting, and . . .
          exercise which is but voluntary labor.'' --J. Seed.

                Spontaneous joys, where nature has its play, The
                soul adopts, and owns their firstborn away.
                                                  --Goldsmith.
          -- {Spon*ta"ne*ous*ly}, adv. -- {Spon*ta"ne*ous*ness},
          n.

Source : WordNet®

spontaneous combustion
     n : ignition of a substance (as oily rags) resulting from an
         internal oxidation process