A subspace surge or subspace energy surge was a subspace phenomemon which could be created by cloaked vessels.
In 2371, the cloaked USS Defiant created a large subspace surge, which was detected by Deep Space 9's security sensors. (DS9: "The Search, Part I")
Later that year, the USS Mekong, carrying Odo and Garak, detected a subspace energy surge when a Romulan warbird decloaked above the runabout. (DS9: "Improbable Cause")
In 2375, Onquanii's ship also produced a subspace surge which could be detected by USS Voyager's sensors. (VOY: "Warhead")