spontaneous as in happening or arising without apparent external cause

• "spontaneous laughter"
• "spontaneous combustion"
• "a spontaneous abortion"

Synonyms

• self-generatedhappening or arising without apparent external cause; originating from the self

spontaneous as in said or done without having been planned or written in advance

• "he made a few ad-lib remarks"

Synonyms

• ad-libremark made spontaneously without prior preparation; perform without preparation; with little or no preparation or forethought; said or done without having been planned or written in advance
• unwrittenbased on custom rather than documentation; using speech rather than writing; said or done without having been planned or written in advance