in the meantime vs meanwhile

in the meantime is not a noun while meanwhile is a noun.

Word	Noun	Adjective	Verb	Adverb
in the meantime	No	No	No	Yes
meanwhile	Yes	No	No	Yes

As adverbs, in the meantime and meanwhile are synonyms defined as:

Other synonyms of in the meantime include meantime.

in the meantime (noun)	meanwhile (noun)
	the time between one event, process, or period and another

in the meantime (adverb)	meanwhile (adverb)
during the intervening time	during the intervening time
	at the same time but in another place