constitutionalism UK /ˌkɒnstɪˈtjuːʃənəˌlɪzəm/US /ˌkɑnstɪˈtuʃənəˌlɪzəm/ noun3 •Adherence to constitutional principles. •Philosophical belief in government under a written constitution. •A constitutional system of governance. formsconstitutionalisms