Decoherence is a phenomenon in quantum mechanics where a quantum system loses coherence, leading to a transition from quantum behavior to classical behavior. This can occur when a quantum system interacts with its environment, causing the system to become entangled with the environment and resulting in a loss of information about the system's quantum state. Decoherence is a significant factor in the study of quantum computing and quantum information theory, as it can limit the ability to create and maintain coherent quantum states. Researchers study decoherence to better understand how quantum systems interact with their environments and to develop strategies for mitigating its effects in quantum technologies.