The term "20mm" typically refers to a measurement of 20 millimeters, which is a unit of length in the metric system. This measurement is commonly used in various fields, including photography, where it can describe the focal length of a lens, and in firearms, where it can refer to the caliber of ammunition, such as the 20mm caliber used in certain military applications. In photography, a 20mm lens is considered a wide-angle lens, allowing for a broader field of view, making it ideal for landscape and architectural photography. In contrast, in the context of firearms, 20mm rounds are often used in anti-aircraft and heavy machine guns, highlighting the versatility of this measurement across different domains.