An onomatopoeic adverb expressing a sudden, loud burst of sound or emotion (e.g., sudden crying, applause, or exclamation).
発表が終わると会場からわっと拍手が沸き起こった。
When the presentation ended, applause suddenly rose from the audience.
その知らせを聞いて、みんながわっと驚いた。
On hearing the news, everyone was suddenly surprised.
試合で決勝点が入ると、観客席がわっと沸いた。
When the winning goal was scored, the stands erupted (with cheers).