Spontaneous (adjective)

1. Happening or arising without apparent external cause.
2. Performed or occurring as a result of a sudden inner impulse or inclination and without premeditation.
3. Natural and unaffected.

Origin:

Late 16th century: from modern latin spontaneus, from latin sponte "of one"s own accord", from sponte sua "of its own accord".

Examples:

1. She had a spontaneous idea to go to the beach.
2. The applause was spontaneous and genuine.
3. The fire was caused by a spontaneous combustion.
4. His spontaneous reaction to the news was to laugh.
5. The dance was spontaneous and improvised.