Cloaking barrier

From Memory Alpha, the free Star Trek reference.

A cloaking barrier was a small area located around some of the Spheres in the Delphic Expanse.

While in one of these, the Enterprise suffered power fluctuations and microfractures along the hull. (ENT: "Anomaly") Later, mainly shuttlepods were used to go through cloaking barriers. They were reinforced with trellium-D as Enterprise could not withstand the pressure. (ENT: "Exile")