all at once
Definitions
Suddenly and unexpectedly.
Simultaneously; all at the same time.
Examples
The lights went out all at once, leaving the room in complete darkness.
She tried to explain everything all at once, but I couldn't keep up with her story.
Synonyms
suddenly abruptly all of a sudden in an instant at the same time simultaneously
Antonyms
gradually slowly bit by bit incrementally step by step piece by piece over time