What is the difference between COPD and emphysema?

The two diseases are related and distinct. COPD is a common disease characterized by irreversible restriction of airflow. It is related to the chronic inflammatory response of airways and lungs to toxic particles and gases. Emphysema is not actually a disease. It is a pathological condition. Most common emphysema is caused by COPD, but there are also simple emphysema without COPD. Besides COPD, emphysema can also be caused by many diseases, such as asthma and bronchiectasis. COPD itself can be associated with emphysema or without emphysema. For example, if there is more gas in a room than normal, we can call it emphysema. When there is more gas in the room and there is no air circulation, we can call it COPD. If the room has more gas, but its airflow is unobstructed and there is no airflow obstruction, then there is emphysema and no COPD.