Let's review size terminology. Greek words are used for powers of 10, and the most commonly used quantities are:

kilo    K    roughly a thousand    really 1,024
mega    M    roughly a million     really 1,048,576
giga    G    roughly a billion     really 1,073,741,824
tera    T    roughly a trillion    really 1,099,511,628,776

The prefixes are attached to byte or word. If a computer's main memory is 1 megaword. then it has 1,048,576 individually addressable memory cells, each one word long, however long a word is in this computer. It might be 64 bits, in which case there are about 64 million bits.

On the other hand, the computer might have 8 megabytes, or 8 x 1,048,576 bytes, or 8,388,608 bytes. Beware of using these prefixes with monetary figures, though it is common nowadays. Make sure that if you sign a contract for 8 megadollars they pay you $8,388,608, not a mere 8 million!

Contractions are often used, such as 2Mbyte or 2M, where M stands for mega. Personal computers often have 8M main memories which would have been unthinkably huge just a decade ago when 64K for main memory was considered reasonable. The earliest IBM Personal Computer, unveiled in 1981, had a tiny 16K of main memory! Most memories are not in the terabyte range yet, but will probably be soon.