Define Causal System - Discrete Time Systems?

A causal system is one which produces an output only when there is an input. All physical systems are causal. In general, a causal discrete-time sequence, x[n], or the impulse response, h[k], of a discrete-time system is zero before time 0, that is x[n] = 0, n < 0, or h[k] = 0, k < 0. The input-output relationship of a Linear Time Invariant (LTI) system is given by the convolution sum

Where h[k] is the impulse response of the system. The values of h[k] completely define the discrete-time signal in the time domain.