What is alert fatigue?
The term Alert Fatigue, also known as Alarm Fatigue has been around for decades. It affects many professions, including medical, technical, and construction industries.  According to the Alert Fatigue by Other Names reference, the earliest literature on the topic came during the Israeli Arab conflict.  The practice/understanding of the issue is ancient.  

No, PagerDuty did not invent the term.

Alert Fatigue:


  occurs when one is exposed to a large number of frequent alarms (alerts) and consequently becomes desensitized to them.  


Aesop's Fable about the boy who cried wolf is a classic example of Alert Fatigue.  In Technology and Medical fields it occurs when alarms/alerts occur so frequently (with or without reason) that we become desensitized to them.  The outcome of this desensitization:


  dramatically reduces our reaction to similar subsequent threats.


In other words we are become so used to the alert that we either ignore it or take our time to acknowledge it.

References

Alert fatigue by other names 
Alarm_fatigue
Cry Wolf - When Experience Becomes Fateful