A transition system is a mathematical model used to describe the behavior of systems that change states over time. It consists of a set of states, a set of transitions between those states, and a set of initial states. Each transition represents a change from one state to another, often triggered by specific actions or events. Transition systems are commonly used in computer science, particularly in the fields of automata theory and formal verification. They help in analyzing the behavior of software systems and protocols by providing a clear framework to understand how systems evolve and respond to different inputs.