spontaneous
definition
occurring as a result of a sudden inner impulse or inclination without premeditation or external stimulus
example
Her spontaneous decision to go on a road trip made for an unforgettable adventure.
synonyms
impulsiveinstinctiveunplanned
word origin — from Latin 'spontaneus', meaning 'of one's own accord, voluntary', from 'sponte', meaning 'of one's own will'