In 2004, the UK National Institute for Clinical Excellence defined chronic obstructive pulmonary disease (COPD) as “characterised by airflow obstruction. The airflow obstruction is usually progressive, not fully reversible and does not change markedly over several months. The disease is predominantly caused by smoking.” COPD is the preferred umbrella term for the airflow obstruction associated with the diseases of chronic bronchitis and emphysema. These are closely related to, but not synonymous with, COPD.

Although asthma is associated with airflow obstruction, it is usually considered as a separate clinical entity. Some patients with chronic asthma also develop airflow obstruction that is relatively fixed (a consequence of airway remodelling) and often indistinguishable from COPD. Because of the high prevalence of asthma and COPD, these conditions coexist in many patients, creating diagnostic uncertainty. Other conditions also associated with poorly reversible airflow obstruction include cystic fibrosis, bronchiectasis, and obliterative bronchiolitis. Although these conditions need to be considered in the differential diagnosis of obstructive airways disease, they are not conventionally covered by the definition of COPD.