On a serious note I agree it seems to be either a complete lack of ideas, or the simple act of picking up the pencil to start seems harder than moving a mountain (although I do find that if I actually do pick it up I can get on with it).
I would define it more like the sum of processes that makes you not draw even if you have the time, mediums and health.
It can be the frustration, the motivation, the lack of ideas, a psychological block, an accumulation or other things.
I guess in some ways, for me, I consider it a lack of ideas... but mostly it's just feeling like NOTHING I'm doing is coming out "right". Like, it doesn't matter what I do.. I'm frustrated because it doesn't look as good as I want it to, and to me it just seems like everything I try to draw sucks.
it's mostly the underlying feeling that something just doesn't seem right. it might be coming off wrong, that line looks too thick or thing, or maybe the fist shouldn't psoe like that. i had this happen to me countless times and i can't seem to get rid of the art block issue. perfectionism had a helping hand in this issue too.
I have those, too. There are days when nothing I paint seems to come out right. It feels like every action taken is just the wrong action. When I'm in that state, I don't like to stop drawing, altogether. It feels like stopping will create the habit, "when things get difficult, quit," which I don't endorse in myself.
So I'll doodle, switch to a different project/commission/style, and sometimes will even go out & sit in a park & gesture draw.
Other times I'll open the painting in a new file, and play around with it until I get somewhere. Having it in a new file often takes the pressure off -- if I mess up too badly, there's still the old file sitting on my disk.