A subspace reflection was a phenomenon indicative of a dimensional shift. It was associated with use of the inverter, a folded-space transporter used by Ansata terrorists on Rutia IV.
In 2366, the crew of the USS Enterprise-D realized that the reflection could be tracked using an adaptive subspace echogram. Using this method, the Enterprise crew was able to locate the Ansata base. (TNG: "The High Ground")
It is unclear whether the subspace transition rebound mentioned earlier in the same episode is a special case of subspace reflection, or an independent phenomenon.