The reason why the word "fatigue" is problematic and is to be avoided is that it has no clear definition.
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But this colloquial use of the word "fatigue" is inappropriate when talking about pathological muscle weakness or pathological cognitive dysfunction.
That's true, the definition of fatigue is not always clearly spelled out. And the difference between physical fatigue and mental fatigue is not alway made clear in the ME/CFS definitions.
Muscle weakness or rapid muscle fatiguability on minimal exertion (low muscle stamina) we could call physical fatigue, because you feel that fatigue in your muscles. That fatigue might only be present when you start exerting your muscles. A lot of ME/CFS diagnostic criteria refer to this low muscle stamina.
There is also the physical feeling of the heavy molasses limbs in ME/CFS, which is a whole body-type fatigue that is present even if you do not use your muscles.
And sometimes I find the whole body just feels tired and low on energy, and then if I going for a walk for example, my walking pace will be a lot slower, due to the lack of bodily energy. That would come under physical fatigue also.
This low muscle stamina form of fatigue is not something that a healthy person would really know about. Nor would they know about the molasses limbs feeling.
Mental fatigue however is something that healthy people can relate to. They would just call it fatigue or tiredness.
This everyday fatigue is even researched scientifically, for example in the airline industry, where it is known that pilot cognitive performance and speed of reactions goes down if the pilot has had a long day, and has become tired mentally.
Every healthy person is familiar with the mental fatigue that builds up towards the end of the day. When you are working on a complicated task all day long, at the end of the day you can feel your mind becomes tired and your performance drops. So that's when you call it a day, and hit the sack. When they wake up the next day, a healthy person is refreshed mentally, and all the tiredness from the night before it gone. So they take up the task again with renewed mental clarity and vigor.
The mental fatigue I experience in my ME/CFS is like this end of the day fatigue that healthy people get, only I tend to get it throughout the day, and it definitely becomes worse during PEM.
When I my ME/CFS was entering into severe territory many years back, my mental and physical fatigue were so severe that I could not even keep awake, and I would be sleeping 12 or more hours at night, and then sleeping on and off throughout the whole day, on the sofa. I'd spend the whole day just drifting in and out of sleep, like you might do if you have the flu.
