A unit of electronic digital storage capacity similar to a megabyte equal to 2^20th (2 to the 20th power) bytes. It was created to reduce the confusion of inconsistent capacity reporting of devices like hard drives. The box of a hard drive might say 500MB (megabytes) while your operating system shows the capacity as something around 480MB. The difference is likely due to some space on the disk being used to store file system metadata, and your computer measuring in MiB (mebibytes). The meta data is used to keep track of where your files are, when they were modified, and an assortment of other important information your computer and its software needs to use them.
CDs typically have a capacity of 650 megabytes or 620 mebibytes.
A new, weird and unnecessary unit of data capacity, created by some idiots of IEC and aggressively advertised on Wikipedia. 1 Mebibyte equals 0.9765625 Megabytes and serves no purpose other than confusing people and as an excuse for ripping customers for a few extra bucks.
The big red letters on the box say "512 MB", but those are our brand new Mebibytes! In fact, your flash drive can only store up to 488 old, bad and stinky Megabytes.