Kaddishfrom an Aramaic word meaning "holy", one of the most solemn and ancient of all Jewish prayers. The Kaddish is recited at a grave and on the anniversary of the death of a close relative.
Although the prayer itself contains no reference to death its use in this regard perhaps arose from the belief that saying the praises of God would help the souls of the dead find everlasting peace.
Besides the Mourner's Kaddish, regular Kaddish is recited at every public prayer service.
Source: Rosten, The Joys of Yiddish.