Also see Kilo, mega, giga, tera, peta, and all that.
1) As a measure of computer processor storage and real and virtual memory, a megabyte (abbreviated MB) is 2 to the 20th power bytes, or 1,048,576 bytes in decimal notation.
2) According to the IBM Dictionary of Computing, when used to describe disk storage capacity and transmission rates, a megabyte is 1,000,000 bytes in decimal notation.
According to the Microsoft Press Computer Dictionary, a megabyte means either 1,000,000 bytes or 1,048,576 bytes.
According to Eric S. Raymond in The New Hacker's Dictionary, a megabyte is always 1,048,576 bytes on the argument that bytes should naturally be computed in powers of two.