simultaneously; all at once; in unison — when many people or things act together at the same time.
掛け声とともに選手たちが一斉にスタートした。
At the shout, the athletes all started at once.
警報が鳴ると、学校の全員が一斉に避難した。
When the alarm sounded, everyone at the school evacuated simultaneously.
ボタンを押すと一斉に電気が消えた。
When the button was pressed, the lights went off all at once.