The new (December 1998)
prefix for 1 048 576. Its
symbol is Mi.
It was created to resolve the confusion between 10^6 and 2^20. The original prefix mega always means 1 000 000, and its symbol is M.
So 1 MB = one megabyte = 1 000 000 bytes.
And 1 MiB = one mebibyte = 1 048 576 bytes.
Likewise kilo is 1000, kibi is 1024. See prefixes for binary multiples for a fuller explanation.