X - X is a ticker symbol extension. The X-extension to is used to identify a mutual fund.
X-efficiency - The degree of efficiency maintained by companies under situations of imperfect competition. When a corporation is said to be efficient, it means that it is maximizing its outputs from all of its inputs, including worker productivity and production efficiency. Firms are required to be as efficient as possible in order to secure significant profits and continuing existence, in a highly competitive market. This is not true when there is insufficient competition, as when there is a monopoly or a duopoly.
XD - The symbol XD is used to indicate that a security is trading ex-dividend. It is an alphabetic qualifier that serves as abbreviation to advise investors about important information regarding a specific asset in a stock quote. At times, when the stock is trading ex-dividend, the symbol X alone might be used to.