meanwhile (n.)

also mean while, late 14c., "mean time, the interval between one specified period and another," from mean (adj.2) "middle, intermediate" + while (n.). From late 14c. as an adverb, "during or in a certain period of time." Properly two words as a noun but commonly written as one, after the adverb.

Others are reading

Definitions of meanwhile from WordNet
meanwhile (adv.)
at the same time but in another place;
meanwhile, back at the ranch...
meanwhile (adv.)
during the intervening time;
meanwhile I will not think about the problem
Synonyms: meantime / in the meantime
meanwhile (n.)
the time between one event, process, or period and another;
meanwhile the socialists are running the government
Synonyms: interim / meantime / lag