I came here looking for the same definition. This site comes up high on the first page of a google search for XDE. As an alternative to the snide and otherwise unhelpful replies to this thread, I thought I'd supply the below. I had to look further and elsewhere to get a definition. I'm going to assume you're looking in a tech forum for the same which happens to be the IBM XDE definition.

"XDE stands for extended development environment; installation is a straightforward matter, as is installation of software on most Windows systems, but it only runs on Windows 2000/NT/ XP and you'll also need Service Pack 2 installed. The entire suite is based on IBM's Eclipse Platform but with added functionality in the UML modeling department. The distribution comes with two CDs - one that installs XDE in IBM's WebSphere Application Developer (WSAD) and one that installs XDE "standalone" with its own copy of WebSphere Workbench (WSW), which is based on Eclipse. For this review I just installed the stand-alone workbench."