Memory and Memory-less System
A system is said to be memory-less, if and only if its output to a corresponding input depends only on the value of the input at that current time.
A system is said to have memory when, for a given input the output does not only depend on the value of the input at that current time.
Causal and Non-Causal Systems
A system is said to be causal if for any input, the output has no dependence on the value of the input at any time in the future. A system is said to be non-causal if for any input, the output has any dependence on the value of the input at any time in the future.
