Online Computer Dictionary

(Or "Mb") megabytes or megabits. When referring to the size or data transfer rate of a storage device which is accessed in multiples of eight bits (e.g. RAM, hard disk) this almost certainly means megabytes, but when referring to the data transfer rate of a communications system it probably means megabits. Some years ago, it is claimed, "MB" always meant megabytes and "Mb" meant megabits but recently this useful distinction has been lost.