A displacement wave is an artificially-generated energy phenomenon, which can be characterized as a polarized magnetic variation.
In 2371, the Caretaker used a displacement wave to bring starships from across the galaxy to his array in the Delta Quadrant. Among the ships pulled to the Delta Quadrant were the Maquis raider, Val Jean, the USS Voyager, a Cardassian Galor-class warship, the Dreadnought missile, and the USS Equinox. (VOY episodes: "Caretaker", "The Voyager Conspiracy", "Dreadnought", "Equinox")
In the year 2267, more than a century earlier, another form of "displacement waves" were detected when the Enterprise was approaching and then in orbit around the planet Gateway the location of the Guardian of Forever. Commander Spock reported to Captain James T. Kirk that something or someone on the planet was causing "turbulent waves of space displacement" that Spock also called "ripples in time". Spock told Kirk he thought it was of very huge scientific importance that they maintain orbit to study the waves. As the landing party would soon discover the waves were caused by the Guardian of Forever, a time portal like device that refused to answer Kirk's question as to whether it was a being or a machine. (TOS episode: "The City on the Edge of Forever")