Closed-loop systems is a concept and methodological approach describing systems that utilize feedback mechanisms, where the output is measured and compared to a desired setpoint to automatically adjust the input. This core characteristic enables self-regulation, disturbance rejection, and adaptive behavior. Research in this domain investigates the dynamics, stability, and performance of such feedback-controlled systems, focusing on how this structure enables precise control and robustness against external disturbances and internal variations. Key characteristics include the continuous monitoring of system output, comparison against a reference signal, and the subsequent adjustment of control inputs based on the observed error, providing the theoretical foundation for achieving automated, stable, and optimal operation across diverse scientific and engineering disciplines.