A cornerback (CB) is a member of the defensive backfield or secondary in gridiron football. Cornerbacks cover receivers most of the time, but also blitz and defend against such offensive running plays as sweeps and reverses. They create turnovers through hard tackles, interceptions, and deflecting forward passes.

Other members of the defensive backfield include strong and free safeties.

The cornerback position requires speed, agility, strength, and the ability to make rapid sharp turns. A cornerback's skill set typically requires proficiency in anticipating the quarterback and wide receiver, backpedaling, executing single and zone coverage, disrupting pass routes, block shedding, and tackling. Cornerbacks are among the fastest players on the field. Because of this, they are frequently used as return specialists on punts or kickoffs.

The cornerback's chief responsibility is to defend against the offense's pass. The rules of American professional football and American college football do not mandate starting position, movement, or coverage zones for any member of the defense. There are no "illegal defense" formations. Cornerbacks can be anywhere on the defensive side of the line of scrimmage at the start of play, although their proximity, formations, and strategies are outlined by the coaching staff or captain. Examples of cornerbacks in the NFL are Jalen Ramsey, Patrick Surtain II, Marlon Humphrey, Jaire Alexander, Sauce Gardner, L'Jarius Sneed, and Charvarius Ward.

Most modern National Football League defensive formations use four defensive backs (two safeties and two corners); Canadian Football League defenses generally use five defensive backs (one safety, two defensive halfbacks, and two corners). A cornerback's responsibilities vary depending on how the defense assigns protection to its defensive secondary. In terms of defending the run, often corners may be assigned to blitz depending on the coaching decisions in a game. In terms of defending passing plays, a corner will be typically assigned to either zone or man-to-man coverage.

The most effective cornerbacks are typically called "lockdown corners". These elite defenders cover an offensive receiver so effectively on either side of the field that the quarterback does not target the receiver being covered.

In zone coverage, the cornerback defends an assigned area of the field. Many schemes and variations were created to provide defensive coordinators great latitude and flexibility which aim to thwart offensive schemes.

When a team is using zone coverage, some areas of the field require special attention when defending against specific pass plays. They include the flats (to defend the screen pass and hitch routes), mid range zones including the void (to defend the "stop n go", quick post, fade, hook, curl, and "sideline" or "out" routes), and finally the deep zones (to defend the post/deep post, chair, streak, "fly", "go", bomb, or Hail Mary routes). These are basic terms (perhaps the most generic) for the basic zones and routes which vary system to system, league to league, and team to team.