Definition of SYMPATHETIC NERVOUS SYSTEM. : the part of the autonomic nervous system that contains chiefly adrenergic fibers and tends to depress secretion, decrease the tone and contractility of smooth muscle, and increase heart rate — compare parasympathetic nervous system.See sympathetic nervous system defined for kids.ADVERTISEMENT.efinition of SYMPATHETIC NERVOUS SYSTEM. : the part of the autonomic nervous system that contains chiefly adrenergic fibers and tends to depress secretion, decrease the tone and contractility of smooth muscle, and increase heart rate — compare parasympathetic nervous system. See sympathetic nervous system defined for kids. ADVERTISEMENT.