At the time the original genre was coined - Progressive House was a restructure of classic house motifs and tempos with a Trance melodic overlay. It was progressive from the standpoint that it wasn't a norm (again at the time). A lot of music styles were eventually lumped into the genre. It was the use of sounds and melodic constructs that was "progressive" at the time -- a departure from the rave / disco / acid sounds that permeated house music productions.
Gabriel & Dresden, William Orbit, Sasha & Digweed, and music on labels like Silver Planet or Hooj Choons typified what would become the blended boundary of House and Trance in the more modern sense. Progressive House tends to be the blend of the two and yet still be distinctive enough you would know it wasn't straight-up either genre.