PID control: What does it mean that an ideal derivative term is not causal?
Causality implies that the current system is only determined by the past state of the system - that is, a causal system cannot depend on its future state. 

Calculating the actual derivative requires knowing the state (infinitesimally) before and after the current moment. This need to define the derivative based on knowledge of the future violates causality. Approximations of the derivative must be used in a practical implementation of a PID controller.