Observability
Observability is a term from control theory.
“In control theory, observability is a measure of how well internal states of a system can be inferred by knowledge of its external outputs. The observability and controllability of a system are mathematical duals.” - Wikipedia (https://en.wikipedia.org/wiki/Observability)
In the world of software products and services, observability means you can answer any questions about what’s happening on the inside of the system just by observing the outside of the system, without having to ship new code to answer new questions. Observability is what we need our tools to deliver now that system complexity is outpacing our ability to predict what’s going to break.
The most difficult form of observability is [distributed] [tracing] within and between application services.