A Shiv (from the Romani word chiv), or shank is an impromptu knife or other stabbing weapon made with any materials available at hand. They can be fashioned from toothbrushes, rib bones, pieces of bed springs, etc. Shivs or shanks are typically a part of a prison inmate's "arsenal", and are usually used once, then disposed of.
In the past, a weapon of this type was called a shiv. It was only called a shank if it was made from the metal "shank" of either a prison shoe, or bed spring. Shank is a verb used to denote the actual use of the weapon. In modern times, however, the word shank is often used in county jails and some prisons to describe any stabbing weapon as well as the act of stabbing.
At some point during his incarceration, Connor O'Brien ("Public Enemy") was given a shiv by a fellow convict (having claimed during his arrest that "cop killers" are treated like kings in prison) and used it to stab his father, another inmate, to death.
Shivs were used by Patrick Butler and his accomplices during their murders inside the Reagan Federal Penitentiary. Shivs were later used by Weeks and his own accomplices during an unsuccessful prison riot. ("Lockdown")
During Reid's imprisonment, a number of shivs were seen. First, they were used by Milos and his gang in an attempt to kill Reid. ("Alpha Male") Then, Frazier and Duerson used a shiv while confronting Luis Delgado, but they never used it against him. ("Assistance Is Futile") Frazier later slashed Luis's throat with a shiv and left him to bleed to death. ("In the Dark")