Mega officially means 1 000 000.
To resolve the confusion with 1024 * 1024 in data processing, in December 1998 the International Electrotechnical Commission created new prefixes for binary multiples (q.v. for fuller explanation).
In brief: kibi is 1024, mebi is 1 048 576, then gibi, tebi, pebi, ebi. Their symbols have a capital letter then i: thus KiB = kibibyte, MiB = mebibyte, GiB = gibibyte, etc.