K, M and MM stand for
What does M and MM stand for?What should be the abbreviation for million, M or MM?

Definition of M

The Roman numeral M is often used to indicate one thousand.
Example of M
An expense of $60,000 could be written as $60M. Internet advertisers are familiar with CPM which is the cost per thousand impressions.
The letter k is also used represent one thousand. For example, an annual salary of $60,000 might appear as $60k instead of $60M.

Definition of MM

The Roman numerals MM are used to indicate one million.
Example of MM
Sales of $3,000,000 might be written as $3MM.
I have seen one million represented by mn and also by m (both lower case).
Hence, you might see ,400,000 expressed as .4 million or .4MM or .4mn or .4m.

In terms of finance, MM typically stands for Millions.
$100 MM mean $ 100 Million.
Here is an interesting fact.
Lowercase "mm" does mean millimeters, but uppercase "MM" is used for millions. "Mille" is Latin for "thousand".
MM is Mille Mille. So technically MM would be "thousand thousands" or "a million" .
I have seen $100M also which used to mean 100-million-dollars but in that example one uses a currency symbol AND an uppercase M. (This is common in financial uses.)
I would also add that the context of the sentence would add to the meaning of the abbreviation used.