| X | In economic models involving international trade, X is usually chosen to represent exports, and M to represent imports, perhaps because E and I have too many other uses. |
| X-efficiency | The ability of a firm to get maximum output from its inputs. Failure to do so, called X-inefficiency or technical inefficiency, may be due to lack of incentives provided by competition. Improvement in X-efficiency is one hypothesized source of gains from trade. Term is due to Leibenstein (1966). [Source] |