Patricide was the act of killing one's own father.
In 2268, after James T. Kirk learned that Spock had taken command following his own injuries at the hands of Thelev, the captain was concerned, as it now made Spock unavailable to give his father Sarek the blood transfusion that would enable the life-saving surgery (a cyrogenic open-heart procedure) Sarek required. Despite Kirk's admiration of Spock's devotion to his duty, he vowed he wouldn't let the first officer commit what was, in effect, patricide. (TOS: "Journey to Babel")
Following the assassination of Klingon Chancellor Gorkon and the naming of his daughter Azetbur as his successor, Montgomery Scott was willing to bet that "that Klingon bitch killed her father." He cited the fact that "They don't place the same value on life that we do," adding that following his death, "She did not shed one bloody tear." Spock, however, remained unconvinced, stating that his observation was "hardly conclusive [...] as Klingons have no tear ducts." (Star Trek VI: The Undiscovered Country)