immediately vs forthwith

immediately and forthwith both are adverbs.

Word	Noun	Adjective	Verb	Adverb
immediately	No	No	No	Yes
forthwith	No	No	No	Yes

As adverbs, immediately and forthwith are synonyms defined as:

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

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

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