The "dime" defense (4 cornerbacks), lined up against 4 wide receivers on offense. A conventional Dime formation would have 4 linemen and only one linebacker.
In American football, a dimeback is a cornerback who serves as the sixth defensive back (fourth cornerback) on defense. The third cornerback on defense is known as a nickelback. The dimeback position is essentially relegated to backup cornerbacks who do not play starting cornerback positions. Dimebacks are usually fast players because they must be able to keep up on passing plays with 3+ wide receivers.
Dimebacks are brought into the game when the defense uses a "Dime" formation, which uses six defensive backs rather than four or five. Usually, a dimeback replaces a linebacker in order to gain better pass defense.