1. to perform an action without planning, preparation, or deliberation; to react naturally and instinctively
She decided to act spontaneously and book a flight to Paris without thinking twice.
Ela decidiu agir espontaneamente e comprou uma passagem para Paris sem pensar duas vezes.
2. to behave in a natural, unforced manner without self-consciousness
The children acted spontaneously during the improv game, creating funny moments.
As crianças agiram espontaneamente durante o jogo de improviso, criando momentos engraçados.