meantime

Meantime and meanwhile have both been used as nouns in prepositional phrases like "in the meantime/meanwhile" since their earliest use in the 1300s. Both have been used as adverbs, like in "meanwhile/meantime, down at the farm…," since the late 1500s. They've been interchangeable for pretty much all of their long histories. But that doesn't mean they don't each have their favorite territory.

Meantime is the one that's usually used as a noun—that is, as the object of the preposition in phrases like "in the meantime" and "for the meantime":

We're headed to the beach this afternoon. In the meantime, I'm going to the grocery store.

And meanwhile is the usual choice for the role of adverb:

The kids are excited about the beach. Meanwhile, someone has to think about dinner.

But you'll see and hear examples that are counter to these generalizations, like the following:

In the meanwhile, someone should find the beach towels.

The baby is napping. Meantime, the older kids are looking for beach toys.

"In the meanwhile" has been accused of being "unidiomatic" (i.e. of sounding weird), but it isn't so unidiomatic that native speakers instinctively avoid it. If you use it, there's no reason you should stop.

But if you want to use this pair in the ways they're most often used and need help remembering which goes where, you can think of this sentence:

In the time it takes to say "in the meantime," you could just as well say "meanwhile."