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1. Happening or arising without apparent external cause
   "spontaneous laughter"; "spontaneous combustion"; "a spontaneous abortion"
   - self-generated
    
2. Said or done without having been planned or written in advance
   "he made a few spontaneous remarks"
   - ad-lib, unwritten

See also: gut, impulsive, instinctive, instinctual, intuitive, natural, spontaneity, spontaneously, unprompted, unscripted

Antonym: induced

Encyclopedia: Spontaneous