Meanwhileas an adverb has two meanings: “during the time something else happens” and “at the same time.” As a noun, meanwhilemeans “the time something else happens.”
Susan went shopping on Saturday; meanwhile, her husband got a tattoo!
We’re moving to California next month, so meanwhilewe’ll be busy packing.
The teacher will be gone for twenty minutes. In the meanwhile, please work on your essays.
"Meanwhile, back at the ranch" is a well-known phrase in US English. In the past it was used by narrators of American cowboy movies as a transition between scenes. Now it is mostly used ironically. Example: "My wife is on a business trip in Hawaii. Meanwhile, back at the ranch, the kids are driving me crazy!"
"In the meantime" is a synonym for meanwhile or "in the meanwhile."
Meanwhile first appeared in Middle English 1300–1350.