X - Exports; the value of all domestic production sold abroad.

XBRL -  eXtensible Business Reporting Language.  It is one of a family of "XML" languages which is a means of communicating information between businesses and on the internet.

X-inefficiency - The use of resources at a lower level of productivity than is possible, even if they are allocated efficiently, so that the economy is at a point inside its production possibility frontier.

X-M- See net exports.