Definition
Happening or done at the same time; immediately.
同時
Everyone stopped working at once when the alarm went off.
警報響起時，所有人立即停止了工作。
She had to make a decision at once because there was not much time left.
她必須立即做出決定，因為沒有太多時間了。
He burst out laughing at once when he heard the news.
他聽到這個消息時，立刻大笑起來。