What does "a template argument may not reference a non-external entity" mean?

What does it mean when GCC (4.9.x with --std=c++11) refuses a specific instantiation of a template, telling me:

/myfile.cpp(28): error: a template argument may not reference a non-external entity

what is a "non-external entity"?

Note: I'm not asking about a specific piece of code. I'm not asking why I got this message for my code. I just don't understand what this message means.