Punishment that is extremely excessive in relation to the crime, shocking to ordinary sensibilities, or equivalent to torture. It is prohibited, but not defined, by the Eighth Amendment to the U.S. Constitution.The definition of cruel and unusual punishment changes over the years, as the courts respond to "evolving standards of decency." The U.S. Supreme Court has ruled that the death penalty, if it could be meted out by juries with wide discretion and little guidance or applied to insane or mentally retarded defendants, is cruel and unusual punishment. There is still much debate about whether certain methods of carrying out the death penalty, including lethal injection, violate the Eighth Amendment's ban on cruel and unusual punishment.
For more detail, see The Meaning of Cruel and Unusual Punishment.