A unique series of letters assigned to a security for trading purposes. NYSE and AMEX listed stocks have three characters or less. Nasdaq-listed securities have four or five characters. If a fifth letter appears, it identifies the security as other than a single issue of common stock. They are also known as "ticker symbols". Taobiz explains Stock Symbol The fifth letter of the ticker symbol of a Nasdaq listed stock can prove very helpful in finding out the history of a company. For example, if there is a "E" as the fifth letter, you know that the company has had previous trouble with their SEC filings.