This paper develops a classification of transition behaviors for hybrid dynamic systems. Physical system behaviors often comprise of multiple temporal and spatial scales. A hybrid system model abstracts the operational domain of a continuous system into patches of similar behaviors. Behaviors in each patch are described by simpler models. When state variables cross threshold values, the system makes discrete transitions from one patch to another. We assign physical semantics to the transition behaviors in order to develop a compositional modeling framework and self-consistent simulators for hybrid systems.