What it means
一気に means doing something all at once, in one uninterrupted effort, or in a sudden rush. It commonly modifies an action verb and emphasizes speed, continuity, or a strong increase rather than merely stating that something happened once. It can describe physical movement, consuming or completing something, and sudden changes in amount or intensity.
Usage note
一気に emphasizes an uninterrupted or sudden progression. It is different from 一度に, which more neutrally means “at one time” or “all together” and often focuses on quantity or simultaneity.
How to use it
一気にVerb (ます-stem)ました
彼は坂を一気に駆け上がりました。
He ran up the slope all at once.
一気にVerb (た-form)
彼は坂を一気に駆け上がった。
He ran up the slope all at once.