The Terkellians were a civilization in the Delta Quadrant, with whom Kathryn Janeway, captain of the USS Voyager, negotiated a nonaggression pact on stardate 52861.
In 2376, Seven of Nine cited this encounter as evidence of a supposed Federation conspiracy to establish a military infrastructure in the Delta Quadrant. (VOY: "The Voyager Conspiracy")
This species was only mentioned in dialogue.
The stardate given would place Voyager's contact with the Terkellians sometime between the episodes "Someone to Watch Over Me" and "Relativity".