also: いちじに · いっときに
adverb
all at once; at the same time; simultaneously
Describes multiple things happening or being done at the same moment, often with a sense of suddenness or concentration. Commonly used with verbs like 来る, 起こる, 言う, 押し寄せる.
注文が一時に来て、店は大混乱だった。
Orders came in all at once, and the shop was in chaos.
みんなが一時に話し始めたので、何も聞き取れなかった。
Everyone started talking at the same time, so I couldn't catch anything.