immediately vs at once

immediately and at once both are adverbs.

Word	Noun	Adjective	Verb	Adverb
immediately	No	No	No	Yes
at once	No	No	No	Yes

As adverbs, immediately and at once are synonyms defined as:

immediately and at once: without delay or hesitation; with no time intervening

Other synonyms of immediately include directly, forthwith, instantly, like a shot, now, right away, straight off, straightaway.

immediately (adverb)	at once (adverb)
without delay or hesitation; with no time intervening	without delay or hesitation; with no time intervening
near or close by	simultaneously
bearing an immediate relation