What is M referring to when talking about memory size( 4M x 8) In the following paragraph its talking about memory and it throws M into the L X W of memory notation and i'm confused on how 4M = 2^22. Thanks in advance

PARAGRAPH:

Memory is built from random access memory (RAM) chips. (We cover memory in detail in Chapter 6.) Memory is often referred to using the notation length   X   width (L   X   W). For example, 4M   X   8 means the memory is 4M long (it has 4M   =   2^2     X   2^20     =   2^22   items) and each item is 8 bits wide (which means that each item is a byte). To address this memory (assuming byte addressing), we need to be able to uniquely identify 2 ^ 22   different items.
M in this context is just defined to be $2^{20}$, so if you have M items (bits, bytes, whatever), you'll have $2^{20}$ of them. The letter was chosen since it's the first letter in the term mega, meaning a million. Since $2^{10}=1024$ is close to $1000$, we'll have $2^{20}=(2^{10})^2≈(1000)^2=1000000$. M is sometimes called a "binary million".