A mebibyte is (210)2 bytes; that is, 1,048,576 bytes. By contrast a meGAbyte is exactly 1,000,000 bytes.
These definitions were created by the IEC and endorsed by the IEEE and CIPM. So far very few people use them. Your neighborhood geek will insist that a megabyte is 1,048,576 bytes. Now you know better!
Reference: Amendment 2 to IEC International Standard IEC 60027-2: Letter symbols to be used in electrical technology - Part 2: Telecommunications and electronics, January 1999.