Spontaneous

внезапен

vnezapen

(sudden, without warning)

естествен

estestven

(produced without being planted or without human labor)

непринуден

neprinuden

(proceeding from natural feeling or native tendency without external constraint)

самопроизво́лен

samoproizvólen

(self generated; happening without any apparent external cause)

спонтанен

spontanen

(done by one's own free choice, or without planning)

спонтанен

spontanen

(arising from a momentary impulse)