1. A powerful new markup language for designing Web pages; an alternative to the older HTML, allowing programmers to define their own markup tags, or formatting commands. From I-gloss

2. A widely used system for defining data formats. XML provides a very rich system to define complex documents and data structures such as invoices, molecular data, news feeds, glossaries, inventory descriptions, real estate properties, etc. As long as a programmer has the XML definition for a collection of data (often called a "schema") then they can create a program to reliably process any data formatted according to those rules. From Matisse