In 1960 Rudolf E. Kálmán introduced a characterization he called observability to describe mathematical control systems. In control theory, observability is defined as a measure of how well the internal states of a system can be inferred from knowledge of its external outputs.
To Observe:
To notice someone doing something, or to notice something happening
To watch or study someone or something with care and attention in order to discover something
To watch it carefully with the hope of arriving at a judgment.
