It's about mebibyte (2^20 bytes, unit symbol MiB), compared to megabyte (10^6 bytes).

For example a harddisk with a terabyte capacity does not have space for 2^30=1,099,511,627,776 bytes but about 10% less. Some disk management tools however were based on base 2 measurements, and the customer might cry foul because the tool reported only about 930 gigabytes, err gibibytes.