It stands for permanent generation:

The permanent generation is special
because it holds meta-data describing
user classes (classes that are not
part of the Java language). Examples
of such meta-data are objects
describing classes and methods and
they are stored in the Permanent
Generation. Applications with large
code-base can quickly fill up this
segment of the heap which will cause
java.lang.OutOfMemoryError: PermGen no
matter how high your -Xmx and how much
memory you have on the machine.