What does M and MM stand for?

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 $1,400,000 expressed as $1.4 million or $1.4MM or $1.4mn or $1.4m.