A subspace instability is a subspace phenomenon that is left as a result of a wormhole opening.
In 2373, the USS Voyager bombarded a subspace instability near the Takarian planet, left from the last appearance of the Barzan wormhole, with verterons. This amplified and polarized the instability, drawing the wormhole back to re-open at that location like a magnet affecting a compass. (VOY: "False Profits")