In Star Trek: Voyager, the experimental weapon used against the Borg in “Endgame” is the transphasic torpedo.
Admiral Kathryn Janeway brings this technology from the future and gives it to the crew of Voyager. The torpedoes are powerful enough to destroy multiple Borg vessels with single impacts, making them far more effective than Voyager’s standard photon torpedoes.
The technology is introduced during the series finale, “Endgame,” which premiered in 2001. Janeway uses future knowledge and equipment to help Voyager survive its final confrontation with the Borg and reach the Alpha Quadrant.
Transphasic torpedoes are often confused with quantum torpedoes, but the two are different fictional weapons. Quantum torpedoes are standard advanced weapons used by Starfleet ships such as the USS Defiant, while transphasic torpedoes are specifically associated with Voyager’s future technology.