A Borg algorithm was an advanced form of computer algorithm using Borg knowledge.
In 2374, Seven of Nine analyzed the sensor logs of USS Voyager using Borg algorithms to determine the number of Omega molecules detected. (VOY: "The Omega Directive")
Later that year, Kathryn Janeway hoped that Seven had some Borg algorithms at hand to decipher an encoded Starfleet message received earlier that year. (VOY: "Hope and Fear")
In 2376, Seven enhanced Voyager's command sequencers with Borg algorithms, which led to malfunctions in secondary systems such as doors and internal communications. (VOY: "One Small Step")