In American football, a 4–3 defense is a defensive alignment consisting of four down linemen and three linebackers. It is called a "base defense" because it is the default defensive alignment used on "base downs" (1st and 2nd downs).

One of the most widely used formations is the 4-3 defense, which features four down linemen and three linebackers. This alignment offers a balanced approach, providing both robust run defense and versatile pass coverage options. Teams often rely on the 4-3 defense to control the line of scrimmage and apply consistent pressure on the quarterback.