A causal system is a system with output and internal states that depends only on the current and previous input values. This property is referred to as causality. A system that has some dependence on input values from the future (in addition to possible dependence on past or current input values) is termed an acausal system, and a system that depends solely on future input values is an anticausal system. However, some authors have defined an anticausal system as one that depends solely on future and present input values or, more simply, as a system that does not depend on past input values.

Classically, nature or physical reality has been considered to be a causal system. Though causality is still thought to be found in nature, discoveries in modern physics have challenged the view that nature is strictly causal. For an in-depth discussion, see causality (physics).

Control theory | Digital signal processing | Systems theory | 因果系统