›› Definition: Megabyte. A megabyte is a unit of information or computer storage equal to 1,048,576 bytes. The official SI definition uses the mebibyte or MiB unit to represent 2 20 bytes.However, most people have requested the more common usage, so the non-SI version is used on this site. megabyte is a unit of information or computer storage equal to 1,048,576 bytes. The official SI definition uses the mebibyte or MiB unit to represent 2 20 bytes. However, most people have requested the more common usage, so the non-SI version is used on this site.