2.3.2 Causality and Realizable Systems
A system in which time is the independent variable is called causal if the output depends only on the present and past values of the input. If y(t) is the output, then y(t) depends only on the input x(t) for values of t < t. Such a system is often referred to as being nonanticipative, because the system output does not anticipate future values of the input.