A causal system is a system in which the output depends only on past and current input values, not on future input values. In other words, the output at any given time is determined solely by the input that has already been applied up to that time. This means that a causal system does not have any knowledge or influence of events that have not yet occurred. For example, the [[unit delay]] system: $ y(n) = x(n-1) $ is causal. The [[impulse response]], $h(n)$, of a causal system verifies the condition: $ h(n) = 0, \, n \lt 0 $