adverb
suddenly and loudly; burst out
Describes a sudden, loud outburst of sound or emotion, such as crying, laughing, or shouting. Often used with verbs like 泣く, 笑う, or 叫ぶ.
赤ちゃんがわっと泣き出した。
The baby suddenly burst out crying loudly.
会場からわっと笑い声が起こった。
A loud burst of laughter erupted from the venue.