What is an antonym for meanwhile?

There isn't one perfect antonym for "meanwhile," as its meaning is quite broad. However, depending on the context, you could use words like:

Simultaneously: If you want to express that something happens at the same time.

At the same time: A more informal version of "simultaneously."

Concurrently: A more formal version of "simultaneously."

Immediately: If you want to express that something happens right after.

Later: If you want to express that something happens after a period of time.

Beforehand: If you want to express that something happens before another event.

Subsequently: If you want to express that something happens after another event.

The best antonym for "meanwhile" will depend on the specific situation and the meaning you want to convey.