A state diagram is a visual representation used in computer science and engineering to illustrate the various states of a system and the transitions between those states. It helps in understanding how a system behaves in response to different events or conditions. Each state represents a specific condition or situation, while arrows indicate the transitions triggered by events. State diagrams are commonly used in the design of software applications, particularly in modeling the behavior of objects in object-oriented programming. They can also be applied in embedded systems and protocol design, providing a clear framework for analyzing complex systems and ensuring they function as intended.