In American football, a nickelback is a cornerback who serves as the fifth (in addition to the typical four) defensive back on the defense. A base defense contains four defensive backs, consisting of two cornerbacks, and two safeties. Adding an extra back makes five, hence the term "nickel", which is the name for 5-cent coins in the United States and Canada. Usually the nickelback will take the place of a linebacker, so if the team were to be in a 4–3 formation, there would now be four linemen, only two linebackers and five defensive backs creating a 4-2-5 formation. However, some teams will replace a lineman rather than a linebacker, creating a three linemen, three linebacker and five defensive back alignment, a 3–3–5 formation. If an offensive team always uses three or more wide receivers, a defense may turn to a nickel defense for their base package on most plays. Usually extra defensive backs, such as a nickelback, are substituted into the defense in situations where the opposing offense is likely to attempt a forward pass, such as 3rd-and-long, or when extra receivers are substituted into the opposing offense.
The nickelback is the third cornerback on the depth chart. The nickelback is not considered a starting position because the starting formation for a defense has only two cornerbacks. Defensive formations with three or more cornerbacks are used often enough that a nickelback will usually see moderate playing time (particularly in the modern, pass-oriented NFL) as well as subbing in for the starting corners.