Does the term "writer's block" refer to the condition in which the author not having anything to write, or has the material but unable to put it in words in the way he wants; or both?
In my opinion, it can be both - the clear indication is that the author has a desire and makes some effort to write - but can't.
This can be opposed to an author on a "sabbatical", who maybe thinks it's a nice idea to finish some of the works or start a new one, but makes no effort to do so. Procrastination, imho, is not a writer's block, however some people may think otherwise.
I'd say it can be both, and that people using the term don't diiferentiate between the two meanings.
I'm not an author, but I do write software. I've been doing that for a very long time.
I've had days where I would write the structure of some program or class or whatever, then just sit there and stare at it and want to puke because it looked just like what I did yesterday or the week before. You want to get on to the interesting part of implementing new functions, and here's this pile of boiler plate that you have to slap down before you can get started. It sits there, staring at you and daring you to do something new - and your mind just goes blank. I have to find something else to work on for a while (fix a bug, or track down the cause of a problem somewhere) before I can face that screen full of mocking boiler plate again.
I'd imagine authors get the same thing, as well as having the other problem - the need to write to meet a deadline (or other reason,) and there's not a single damn idea or word to be had.
Another way I have of dealing with that screen full of mocking boiler plate is to not write it. Scribble the interesting parts together first, then go back and pack them in the needed boiler plate junk so that you can use it.