The Klingon term for the companion who stands with and fights for an accused person before the High Council, a role Picard fills for Worf in Sins of the Father.
A cha'DIch is the second chosen by a Klingon who must answer a challenge before the High Council. Because the accused is denied the right of combat while the charge stands, the cha'DIch defends him against any attack and stands beside him throughout.
The custom appears in The Next Generation episode Sins of the Father. Worf goes to the Klingon homeworld to answer the charge that his father Mogh betrayed the outpost at Khitomer, and names as cha'DIch his newly discovered brother Kurn, whose parentage is kept secret.
When Kurn is ambushed and badly wounded by assassins, Worf asks Picard to take his place. Picard accepts, fights off an attack of his own, and finds the old nurse Kahlest, whose testimony forces Chancellor K'mpec to admit that the true traitor was the father of Duras.