The Khitomer system was a star system containing the inhabited planet Khitomer. It was the location of a Klingon colony which fell victim to a Romulan attack in 2346. (TNG: "Sins of the Father")
As of 2369, the Khitomer system was located within the Romulan Star Empire. (TNG: "Birthright, Part II")
The name of this system appeared in a display readout that appeared in "Sins of the Father" but was not referenced in dialog.