In the sport of football, the "Nickel 40" refers to a specific type of defensive scheme used by a defensive unit. It combines the use of five defensive backs (the "nickel") with four defensive linemen (the "40"). This leaves two linebackers, one of which will be used for pass coverage.
Because one linebacker will be in coverage, he is liable to be replaced by a more adept defensive back, although that would technically not be a nickel defense anymore.
The Nickel 40 is a solid base defense that is designed to prevent pass completions without giving up too much on the ground. Ideally, it is used in a blitz against slow-developing plays, such as play actions.
The Nickel 40's main weakness, though, lies in defending the run game. Even though it includes four down linemen, the lack of a third (and sometimes second) linebacker can be problematic when defending against the run.