A jury was a group of individuals responsible for deciding whether an accused criminal was guilty or innocent in a court of law.
Klingon trials had no jury. (ENT: "Judgment")
In 2153, Bethany bemoaned the fact that those responsible for lynching a Skagaran would have also sat on the jury if a trial had been held. (ENT: "North Star")
In 2364, Q reminded Jean-Luc Picard that he was also the jury, in addition to being the judge and prosecutor. (TNG: "Encounter at Farpoint")
In 2374, Julian Bashir criticized Section 31's autonomy, claiming their apparent functioning as judge, jury, and executioner (metaphorical or literal) was too much power for anyone. (DS9: "Inquisition")