'at once' definitions:

Definition of 'at once'

From: WordNet
adverb
Without delay or hesitation; with no time intervening; "he answered immediately"; "found an answer straightaway"; "an official accused of dishonesty should be suspended forthwith"; "Come here now!" [syn: immediately, instantly, straightaway, straight off, directly, now, right away, at once, forthwith, like a shot]
adverb
Simultaneously; "he took three cookies at a time" [syn: at a time, at once, at one time]

Synonyms of 'at once'

From: Moby Thesaurus