What does "all at once" mean?

Unexpectedly; without warning; all of a sudden.

What is a synonym for "all at once"?

Common synonyms include: abruptly, all of a sudden, all together, all together, at a blow, at a stroke.

Pronounced //ˌɔːl ət ˈwʌn(t)s//.

/ˌɔːl ət ˈwʌn(t)s/ adv

adv ·Uncommon ·Advanced level

Adverb

1
Unexpectedly; without warning; all of a sudden. idiomatic, not-comparable
"When all at once I saw a crowd, / A host, of golden daffodils"
2
Used other than figuratively or idiomatically: All at the same time; all together. not-comparable
"There are too many to get in the lift all at once, so some must wait."

Adverb

"When all at once I saw a crowd, / A host, of golden daffodils"