Noah Webster's New International Dictionary of the English Language

1. (a.) Impulsive; extemporaneous; proceeding from natural feeling, temperament, or disposition, or from a native internal proneness, readiness, or tendency, without constraint; as, a spontaneous gift or proportion.

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

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