Definition of 'M'
The fifth letter of a Nasdaq stock symbol that specifies the issue is the firm's fourth class of preferred shares. Securities that are listed on the Nasdaq exchange have four or five letters.
When a fifth letter appears, it identifies the issue as being one that is other than a single issue of common stock or capital stock. A "P" indicates that the issue is the company's first class of preferred shares; an "O" indicates that the issue is the company's second class of preferred shares; "N" indicates the third class of preferred shares. "M" indicates the fourth class.
Investopedia explains 'M'
Preferred shares give holders a class of ownership that has a higher claim on assets and earnings than holders of the company's common stock. Dividends for preferred shares are paid out before dividends to common stockholders.
However, preferred shareholders generally do not have any voting rights. Dividends for preferred shares are fixed; that is, they do not fluctuate like the dividends for the company's common shares. Nasdaq symbols utilize other fifth-letter identifiers such as "A" for class A shares; "B" for class B shares, "C" for issuer qualification exception; "D" for new issue; "E" for delinquent.