Something spontaneous happens when you're least expecting it. Spontaneous things are natural or instinctive, and they happen without warning.

If you plan to go to a movie and then you go, that's not a spontaneous thing. Ditching the movie at the last minute to go to the park? That's spontaneous. Spontaneous things happen all of a sudden, on the spur of the moment, without being planned. Spontaneous can also refer to things that happen without outside influence. Your parents will be happy if you do your chores spontaneously instead of waiting till they nag you.

Definitions of spontaneous

adj said or done without having been planned or written in advance

ad-lib, unwritten
not furnished with or using a script

adj happening or arising without apparent external cause

spontaneous laughter”
spontaneous combustion”
“a spontaneous abortion”
impulsive, unprompted
proceeding from natural feeling or impulse without external stimulus
spontaneously derived from or prompted by a natural tendency
instinctive, natural
unthinking; prompted by (or as if by) instinct
brought about or caused; not spontaneous
elicited, evoked
called forth from a latent or potential state by stimulation
induced by a physician's words or therapy (used especially of a complication resulting from treatment)
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