Meantime is a noun which is used to mean 'an intervening period of time'. Meanwhile can refer to an intervening period of time too, but it can also mean 'at the same time, while something else takes place'. For example; "On Friday I am going to Hong Kong but IN THE MEANTIME, (the time between now and Friday), I will continue going to school" "On Friday I am going to Hong Kong but MEANWHILE I will continue going to school" As stated above, meanwhile can also mean 'at the same time' For example; "I go to school, meanwhile the staff in Hong Kong play golf and drink beer" I hope this helps you.

As adverbs, "meantime" and "meanwhile" are synonyms. The words also can appear as nouns, signifying an intervening period of time. "The train leaves in the morning, but in the meantime let's prepare our material". The big difference between the two is that "meanwhile" gives the idea of something happening at another location. The cowboy movies I watched as a kid always had the line, "meanwhile, back at the ranch…"