Observability

In control theory, observability is a measure for 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. The concept of observability was introduced by American-Hungarian scientist Rudolf E. Kalman for linear dynamic systems.

Read more about Observability:  Definition, Continuous Time-varying System, Nonlinear Case, Static Systems and General Topological Spaces