spontaneous (suomeksi)

spontaani (a: self generated; happening without any apparent external cause / done by one's own free choice, or without planning / not apparently contrived or manipulated)
oma-aloitteinen (a: done by one's own free choice, or without planning)
omaehtoinen (a: done by one's own free choice, or without planning)
impulsiivinen (a: arising from a momentary impulse)
luonnollinen (a: produced without being planted or without human labor)
yllättävä (a: sudden, without warning)
äkillinen (a: sudden, without warning)