Causality
A system is said to be causal if the output $ Y[n] $ at $ n0 $ depends only on the input $ X[n] $ at $ n<no $
In other words
The output signal $ Y[n] $ of a causal system is dependent only on the present and past samples of the input signal, X(n) and not on any future samples of the input
Non-Causality
A system is said to be non-causal if the output $ Y[n] $ at $ n0 $ depends on future values of input $ X[n] $ $ i.e $ at $ n>n0 $
