A megabyte is a completely accurate unit of measuring data.
Well, now that you come to bring up the topic, it's not actually. And in data transmission speed terms is very rarely used.
Strictly it used to be 1000 x 1024 bytes of 1,048,576 bytes but it depends where the term is used. Many systems use 1,000,000. Your hard drive has, say 250MB or space but that's not 250 x 1,048,576 bytes of storage.
"The megabyte is a multiple of the unit byte for digital information. Its recommended unit symbol is MB. The unit prefix mega is a multiplier of 1000000 in the International System of Units. Therefore, one megabyte is one million bytes of information. This definition has
been incorporated into the International System of Quantities."
This is why it's recommended these days that the term MiB (
mebibyte is a multiple of the unit byte for digital information) is now in common usage for the computer related terminology to avoid confusion.