The traditional reliability concepts aim at static logic, which are no longer satisfy the requirement of the modern complex dynamic system reliability analysis. This article provides a different state space and structural function for describing dynamic system reliability. Then concepts of failure sequence and failure cluster are defined for characterizing dynamic systems' failure mode. They are extensions of the traditional static reliability concepts such as cut set and implication set, which are the special example of the provided concepts. The analog concepts including probability importance degree, structural importance degree and critical importance degree are also put forward. At last the relative concepts are illustrated using a simple example of dynamic system.