"Greyed out", also referred to as "Ghosted"
I know this is from Wikipedia, but it seems to state things pretty clearly.
Grayed out
From Wikipedia, the free encyclopedia
In computing, the term grayed out describes a user interface element that has been displayed with a light shade of gray instead of black — or, in the case of colored elements, displayed in a lighter color — to indicate that it cannot currently be operated or selected by the user.
This convention is widely used in graphical user interfaces, to quickly let the user know which actions cannot be taken in the computer's current state. The approach makes it possible to disable a control when it cannot be used, and give a clear signal of that state, while maintaining the control in a consistent location, thereby minimizing confusion for the user.
It says this is a convention, which suggests it is not a formally documented standard, but nevertheless, conventions are very powerful references when they are widely adopted - and I would be hard pressed to come up with something more widely adopted than this!