According to vampire terminology, it is a period in a new vampire's existence when he or she experiences a yearning to return to their mortal life, that makes them extremely agitated and depressed. It is like a vampire's version of puberty. It is experienced by Karl in the episode A Grave Matter, and it because of this that he unwittingly lets himself by seen by Malcolm Frye, which leads to his post on the internet that catches the interest of Marianne Hackett, and almost leads to the vampires being exposed and killed. It is because of the Chill that Karl attempts to reunite with his human friends and his family, but thanks to Drew, he eventually comes to his senses and realizes he can't go back to his old life.