The Roman numeral M is often used to indicate one thousand, and MM is used to indicate one million. For example, an expense of $60,000 might appear as $60M. Sales of $3,000,000 might be written as $3MM. Internet advertisers are familiar with CPM which is the cost per thousand impressions.
In recent years some people began using k to represent one thousand. For example, an annual salary of $60,000 might appear as $60k instead of $60M.
In a recent business publication I saw million represented by mn and also by m (both lower case). This means it is possible for you to see $1,400,000 expressed as $1.4 million or $1.4mn or $1.4m or $1.4MM or $1,400k or $1,400M.