A long and half-forgotten part of the Klingon succession ritual in which claimants recite their deeds, revived by Picard in Reunion to buy time for an investigation.
The ja'chuq is an old element of the Klingon Rite of Succession. In it each challenger for the leadership of the High Council must stand and list his battles and honours at length, to prove he is worthy to lead. By the 24th century it has fallen out of use and is considered obsolete.
It appears in The Next Generation episode Reunion. After a bomb disrupts the Sonchi ceremony for K'mpec, Picard needs time to learn which claimant is responsible. On the advice of K'Ehleyr he announces that, as Arbiter of Succession, he will insist on the full ancient form, including the ja'chuq.
Gowron and Duras are both furious but cannot refuse. The delay allows the Enterprise crew to tie the bomb to Duras's side, although Duras dies at Worf's hand before the rite can be completed.