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1. Happening or arising without apparent external cause
   "spontaneous combustion"; "a spontaneous abortion"; "spontaneous laughter";
   - 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, unprompted, unscripted

Antonym: induced

Encyclopedia: Spontaneous