The term "20mm caliber" refers to a specific measurement of the diameter of a projectile, which is 20 millimeters. This caliber is commonly used in various types of firearms, including anti-aircraft guns, machine guns, and some rifles. The 20mm round is known for its high velocity and significant stopping power, making it effective against armored targets. In military applications, the 20mm caliber is often associated with weapons like the M61 Vulcan and the Oerlikon cannon. These weapons are designed to engage aerial threats and provide ground support. The versatility and effectiveness of the 20mm caliber have made it a popular choice in modern warfare.