Accardi's theorem is a result in the field of quantum probability and noncommutative probability theory. It provides a framework for understanding the relationship between classical and quantum stochastic processes, particularly in the context of quantum Markov processes. The theorem establishes conditions under which certain types of quantum processes can be represented in a way analogous to classical processes. The theorem is named after Francesco Accardi, who contributed significantly to the development of quantum probability. Accardi's theorem has implications for various areas, including quantum information theory and the study of quantum statistical mechanics, helping to bridge the gap between classical and quantum frameworks.