The Dead Zones was a term given by the United Federation of Planets to a number of strange phenomena that affected the galaxy in 2376. The Romulan Star Empire referred to these anomalies as power deserts.
Formation of such anomalies led to the nullification of any higher energy technology output leaving such objects inert. The rapid formation of Dead Zones led to the evacuvation of three Romulan colonies on a massive scale with seven smaller Federation colonies suffering the same fate. Mars nuclear reactor research facility on the Utopia Plateau was also affected. This led to contact being lost with Starbase 244, Starbase 15 and Deep Space 9. In the Delta Quadrant, the USS Voyager and a Borg Cube were also immobilized by a Dead zone. Dead Zones were not simply the end of subspace communications but would have included the end of interstellar exploration with every galactic civilization collapsing completely as high technology became inactive.
There was mounting evidence that showed that the occurences of the power deserts was but one stage in a continous phenomena. Statistics showed that the most recent appearances of the zones allowed more simple technology such as tractor beams and even disruptors to function but as the anomalies "aged" than even these lower level power usages became inactive.
During T'sart's defection to the Federation, he informed the crew of the USS Enterprise-E that he knew what was causing the Dead Zones to appear and how to stop them. He later revealed the source of the disturbance was centered in the Caltiskan system. (TNG novel: Dead Zone)