A subspace eddy was a potentially disruptive spatial phenomenon.
In 2152, the Vulcan starship Seleya became caught in a subspace eddy while attempting to chart the thermobaric clouds at the perimeter of the Delphic Expanse. As a result, the Seleya was pulled into the Expanse, and all contact with the ship was lost. (ENT: "Impulse")
In 2378, the Delta Flyer II passed through a subspace eddy. Captain Kathryn Janeway suspected it was the wake of another ship. The Doctor dismissed it as probably just the gravimetric shear from a nearby Mutara class nebula. The eddy turned out to be the wake of a Hierarchy survey vessel. (VOY: "Renaissance Man")