Emphysema usually refers to a long-term, progressive disease of the lungs that primarily causes shortness of breath. Subcutaneous emphysema refers to a condition when gas or air is present in the subcutaneous layer of the skin.

In people with emphysema, the tissues necessary to support the physical shape and function of the lungs are destroyed. It is included in a group of diseases called chronic obstructive pulmonary disease or COPD (pulmonary refers to the lungs). Emphysema is called an obstructive lung disease because the destruction of lung tissue around smaller sacs, called alveoli, makes these air sacs unable to hold their functional shape upon exhalation. Emphysema is most often caused by tobacco smoking and long-term exposure to air pollution.

The term "emphysema" is derived from the Greek ἐμφυσᾶν emphysan meaning "inflate" - itself composed of ἐν en, meaning "in", and φυσᾶν physan, meaning "breath, blast".

Emphysema can be classified into primary and secondary types. However, it is more commonly classified by location into panacinary and centroacinary (or panacinar and centriacinar, or centrilobular and panlobular).