A defensive back who is substituted into the game when the defensive formation calls for an extra defensive back. The nickel back tends to be the third defensive back on the depth chart and is often better at playing in the middle of the field. Nickel backs can have a large variety of roles including covering receivers, blitzing the quarterback, and guarding against running backs coming out of the backfield.
Nickel backs are an important part of a team's passing defense. As teams switch to passing formations, the nickel back will be called on to replace one of the linebackers in the formation to guard the extra wide receivers. This is essential because typically a wide receiver against a linebacker will be a significant mismatch in favor of the offense. By adding a nickel back, the defense minimizes the advantage the extra receiver gives the offense.