[Definition] Chronic obstructive pulmonary disease (COPD) is a lung disease characterized by airflow limitation, which is not completely reversible and develops progressively. COPD mainly affects the lungs, but can also cause systemic (or extra-pulmonary) adverse effects. Chronic obstructive pulmonary disease (COPD) has become an important public health problem due to its high prevalence, high mortality rate, and high socioeconomic burden. COPD is currently the 4th leading cause of death worldwide, and the World Bank/WHO has announced that COPD will be the 5th leading economic burden of disease in the world by 2020.