Spontaneous means happening naturally without planning or without someone clearly causing it on purpose. When something is spontaneous, it occurs on its own, driven by an inner impulse rather than careful preparation.

A spontaneous decision is one you make in the moment, following your instincts. If your family spontaneously decides to go for ice cream on the way home from school, nobody planned it beforehand: someone just suggested it and everyone agreed. When friends break into spontaneous laughter, they're not forcing themselves to laugh; it just bubbles up naturally because something struck them as funny.

In science, spontaneous describes things that happen by themselves without needing an outside push. Spontaneous combustion occurs when something catches fire on its own, without anyone lighting a match.

People who are spontaneous tend to act on their feelings and ideas without overthinking every detail. They might suggest an impromptu game during recess or decide to try a new route home just to see where it leads.

The opposite of spontaneous is deliberate or planned. A surprise party requires careful planning, but the guests' spontaneous cheers when the birthday person walks in happen naturally, without rehearsal.