Conventional computation models assume symbolic representations of states and actions. Gurevich's Abstract-State Machine model takes a more liberal position: Any mathematical structure may serve as a state. This results in "a computational model that is more powerful and more universal than standard computation models". We characterize the Abstract-State Machine model as a special class of transition systems that widely extends the class of "computable" transition systems. This characterization is based on a fundamental Theorem of Y. Gurevich.