The definition of a reactive system is one that responds to changes in its environment. This type of system is designed to respond quickly and accurately to changes in its environment in order to maintain a desired state. Reactive systems are often used in applications such as robotics, artificial intelligence, and computer vision. These systems are able to react to changes in their environment in order to take the appropriate action. For example, a robot may be programmed to react to changes in its environment by avoiding obstacles or navigating around them. Similarly, an AI system may be programmed to react to changes in its environment by making decisions based on the data it has collected. Reactive systems are designed to be highly responsive and adaptive, making them ideal for applications that require quick and accurate responses.