The problem with asking for definitions, of course, is that the evidence just isn't there in sufficient detail to say whether particular cases fit one definition or another.
That's true of ANY historical event, behavior or effect. Sure it's a problem--it's ALWAYS the problem with historical evidence--but you can make the job of answering the question from the evidence available much easier if you take some time to ask the question(s) effectively. A different question or the same one worded differently calls on different evidence.
For instance, it has always been assumed that the Confederates hid in and behind the cornfield and surprised the Union at Antietam, but there weren't any reports of this fighting 'in the cornfield', just at the edges. It has been 'just recently discovered' that 1862 had been a drought year, so the corn was only about elbow high--something supported by contemporary reports from the area. It wouldn't have hidden anyone standing or kneeling, and all the reports have the Confederates standing or kneeling. Actually, they were at the far end of the field behind a fence broken down into a barracade--kneeling behind it--surprise. Even though they knew Rebels fought at the edge of the cornfield, they assumed they would have naturally used the field as cover. No one had thought to ask how high the corn was. . .. New question, new evidence.
A few years ago, there was a serious discussion about whether the 33rd VA had tried to fool the Union into thinking they were yanks at 1st Bull Run. The issue hinged on whether the Yanks could recognize the Rebel stars and bars, or mistaken it for the stars and stripes. No one thought to ask what flag the 33rd flew. It took me ten minutes at the Manassas Battlefield archives to find that the 33rd didn't have a standard at 1st Bull Run. [Sorry about the ACW examples, but I had those mentally handy.]
I wasn't suggesting a narrow definition, but rather a statement of what this fatigue is you are looking for and then a list of the types of behaviors, comments and results you would expect to see because of it. It's like developing a diagonsis as a doctor, a case as a lawyer, or any kind of conclusion developed from limited information. It starts with asking the right questions.
Happy Holidays and a Wonderful New Year.