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
  1. adjective
    said or done without having been planned or written in advance
    synonyms: ad-lib, unwritten
    not furnished with or using a script
  2. adjective
    happening or arising without apparent external cause
    spontaneous laughter”
    spontaneous combustion”
    synonyms: self-generated
    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
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    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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