spontaneous

Concise Oxford English Dictionary © 2008 Oxford University Press:
spontaneous /spɒnˈteɪnɪəs/

▶adjective

• 1 performed or occurring as a result of an unpremeditated inner impulse and without external stimulus.

  ■ (of a process or event) occurring without apparent external cause.

  ■ Biology (of movement or activity) instinctive or involuntary.

• 2 open, natural, and uninhibited.

• 3 archaic (of a plant) growing naturally and without being tended or cultivated.

– derivatives
spontaneity /spɒntəˈneɪɪti/ noun,
spontaneously adverb.

– origin C17: from late L. spontaneus (from (sua) sponte ‘of (one's) own accord’) + -ous.