What does "all of a sudden" mean?

Quickly and unexpectedly; without warning.

Is "all of a sudden" formal or informal?

"all of a sudden" is register-neutral — appropriate in both casual and formal contexts. Emphasizes the unexpected and abrupt nature of something that happens.

all of a sudden

初級 (A2) neutral

Very suddenly, without warning.

"All of a sudden, the lights went out."

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