1. to respond or act in an immediate, unplanned, or instinctive manner without deliberation or forethought
She reacted spontaneously to the surprise party by hugging everyone.
Ela reagiu espontaneamente à festa surpresa abraçando todos.
2. to respond naturally and instinctively to a stimulus or situation
The crowd reacted spontaneously with laughter at the comedian's joke.
A multidão reagiu espontaneamente com risadas à piada do comediante.