spontaneous

Happening or arising naturally without planning or external cause; done on impulse.

• spontaneously/spɒnˈteɪniəsli/(adv)自发地
• spontaneity/ˌspɒntəˈniːɪti/(n)自发性

• spontaneous applause
• spontaneous combustion
• spontaneous reaction
• act spontaneously
• spontaneous decision

1. 1.The crowd broke into spontaneous applause before the performance had even ended. (unplanned reaction)
2. 2.She's known for her spontaneous travel plans — she once booked a flight to Tokyo with four hours' notice. (impulsive)

• impromptu/ɪmˈprɒmptjuː/— done without preparation, usually for a speech or performance
• impulsive/ɪmˈpʌlsɪv/— acting on sudden urge, often recklessly
• unplanned/ʌnˈplænd/— neutral; simply not planned

from Latin "spontaneus" = of one's own will, from "sponte" = willingly