The Dementor's Kiss is the most terrifying and final act a Dementor can perform. It is not a spell cast by a witch or wizard, but rather an innate and horrific ability of the Dementor creature itself. The Kiss is considered by the Wizarding World to be a fate worse than death.
The Dementor's Kiss was employed by the Ministry of Magic as the ultimate punishment for the most heinous criminals, typically those imprisoned in Azkaban. Its use was heavily controlled, though instances of unsanctioned use have occurred.
There is only one known method of defense against a Dementor advanced enough to perform the Kiss.
The term “Dementor's Kiss” is a grim euphemism.