一斉に|いっせいにisseini

1. adverbsimultaneously; all at once; in unison
   人々ひとびとが一斉いっせいに走はしり出だした。↗

   People started to run all at once.

• Synonym同時にどうじにNeutralRefers to things happening at the same time; its range is broad and it can also mean 'meanwhile'. 「一斉に」 stresses a group acting together in unison.
• RelatedいっぺんにNeutralMeans doing everything in one go, stressing the one-shot, in-one-sitting feel; it does not stress a whole group acting in unison.
• Related一度にいちどにNeutralMeans all at once, with the focus on things happening in a single instance, as in 「一度に食べる」; it does not stress a group acting together.
• Antonym別々にべつべつにNeutralMeans separately, each on their own — the opposite of 「一斉に」, where everyone acts together as one.
• Antonym個別にNeutralMeans dealing with each case or person individually, as in 「個別に対応する」; it contrasts with 「一斉に」 treating everyone uniformly.
• AntonymばらばらにNeutralMeans scattered or in disarray, the opposite of the neat, uniform action that 「一斉に」 describes.

「一斉に」 is for a group or many things acting together in unison, as in 「一斉に立ち上がる」; for mere overlap in time use 「同時に」, and for getting everything done in one go use 「一度に」 or 「一遍に」. For the opposite, 「別々に」 and 「個別に」 mean separately, 「ばらばらに」 scattered.

Usage comparisons are AI-assisted and cross-checked by multiple models; for study reference only

1. 同おなじ時刻じこくに、みんなで一緒いっしょにすること。