The megabit is a unit of information storage, abbreviated Mbit or sometimes Mb.
1 megabit = 10 bits. Residential high speed internet is often measured in megabits.
A binary counterpart of the megabit, useful for measuring RAM and ROM chip capacity, is the mebibit.
Video game cartridges
In the 16-bit games console era, the 'megabit' (Mbit) became a commonly-used measure of the size (data storage capacity) of games cartridges. This "megabit" actually equals to one mebibit, the symbol of which is "Mibit". The vast majority of SNES / Megadrive (Genesis) games were produced on standard 8 Mibit cartridges, although 4 Mibit cartridges were not uncommon and, later in the life of these consoles, graphically-intensive titles would appear on 12, 16, even 32 Mibit cartridges. Tales of Phantasia and Star Ocean were 48 Mibit.
The rule is: 8 bits = 1 byte. Therefore a 4 Mibit cartridge had a capacity of 512 KiB, an 8 Mibit cartridge held 1 MiB of data, and so on.