Neuromodulation is the process or phenomenon by which neural activity is altered, encompassing both endogenous biological mechanisms (e.g., via neuromodulators) and exogenous interventions (e.g., electrical or chemical stimulation). As a key concept and research area, it investigates the mechanisms and functional consequences of these modulatory influences on neural circuits, brain function, and behavior, offering critical insights into physiological processes and pathological states, and serving as a basis for therapeutic approaches to neurological disorders.