What is GDX?

This was a question I was asked earlier today. Someone thought it was an exotic digital mode. It is not.

GDX means "best DX in the UK" , nothing more.

If you live in the UK it is probably a phrase you know. If you live outside the UK then it may be a phrase you've maybe never heard of before or were too embarrassed to ask.

In the same way ODX means "best overall DX".

UPDATE 1545z:  Just 6m locals and GDX this afternoon. No Es here since CN8LI lunchtime.  May be more teatime?