Observability
Watching a set of chosen measurements on systems over time and warning people when one moves outside its normal range.
Formal
The ongoing collection and display of known measures of a system's health, such as whether it answers, how busy it is and how often it fails, checked against set limits so that known kinds of trouble raise an alarm.
In plain English
Like the warning lights on a car's dashboard; they tell you quickly that the oil is low, but not why the engine is making a strange noise.
In practice
A screen in a water utility's control room shows memory use, response times and error counts for the servers that run its pumping stations, and turns red when one of them stops answering.
Why it matters
It keeps services available by catching known problems before users notice, and it is often the first place an attack shows up as odd load or failures.