Please define

If you're referring to listener fatigue, it's a term I personally associate with a very forward, in-your-face type of sound. When the soundstage is pushed aggressively and artificially toward me, I lose all sense of background, sublety, microdynamics, and so on. Over time I find that such sonics become "fatiguing" to listen to.

Your mileage may vary, reasonable minds may differ, your gear and your ear, etc.

I find it really depends to a great extent on the music you are playing and the mood you are in. Sometimes I want it face forward sometimes I just want it sweet. I also find that having a couple of different systems help!! Volume by the way is critical. Your hearing, so I am told, changes with respect to which frequencies you hear best at different volumes. Most fatigue is heard in the treble or upper mids at loud volume.

Oops!, Sorry, yes, the term is "listener fatigue."

I associate listener fatigue with distortion. It's obvious that if you're listening to a system that is relatively harsh, edgy or shrill, your ears soon will find it unpleasant. But frequency imbalances that are not nearly as noticeable in the short term can produce fatigue after a while. There may be shouty, boomy, or ringing parts of the musical spectrum that aren't really obvious but just get to you after a while. The louder you listen, the quicker these shortcomings get to you. The room, as well as the equipment, can produce listening fatigue.