stock symbol

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".

Investopedia Says:
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.

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Letters used to identify listed companies on the securities exchanges on which they trade. These symbols, also called trading symbols, identify trades on the Consolidated Tape and are used in other reports and documents whenever such shorthand is convenient. Symbols for stocks listed on the New York Stock Exchange and the American Stock Exchange range from one to three letters. AT&T's symbol, for example is T; Alcoa's is AA; 3M Company's is MMM. A fourth letter indicates a special class or category. Over-the-counter stock symbols have four or five letters. A fifth letter indicates that the security has something special about it. The following is a guide: A = Class A; B = Class B; D = New; E = Delinquent in SEC filings; F = Foreign, except ADRs; G, H, I = Additional warrants of preferreds; J = Voting; K = Nonvoting; L = Miscellaneous situations, such as stubs, depositary receipts, additional warrants or preferred; M = Fourth preferred; N = Third preferred; O = Second preferred; P = First preferred; Q = bankruptcy; R = Rights; S = Beneficial interest; T = Delinquent in filing; U = Units; V = When-issued and when-distributed; W = Warrants; Y = ADRs; Z = Miscellaneous situations, similar to L. Stock symbols are not necessarily the same as abbreviations used to identify the same companies in the stock tables of newspapers. See also Committee on Uniform Securities Identification Procedures (CUSIP).

The noun has one meaning:

Meaning #1: the letters used to identify listed companies on the securities exchanges where they are traded