The abbreviation used to identify a company on a stock exchange. In the United States, they are between 1 and 5 letters long, depending on the stock exchange, whether there are suffixes, and how cool the company is. NYSE symbols are 1-3 letters, NASDAQ ones are 4 or 5.

Nobody cares about AMEX anymore, since they merged with (read: got eaten up by) the NASDAQ. Not that anybody cared prior to the merger.