Describes an action performed by many people or things at the same time, often in an organized or coordinated manner. Commonly used with に as an adverb.
The students stood up all at once.
The police launched a simultaneous investigation.
On hearing the news, the students all stood up at once.
School closures were announced simultaneously at schools across the country.
volley firing; fusillade; broadside
all-out attack; attack on all fronts; onslaught