The Collaborative International Dictionary of English v.0.48:

groundswell \ground"swell`\ (ground"sw[e^]l`), n.
   1. A long, deep wave in the sea, sometimes caused by distant
      winds or storms.
      [PJC]

   2. A rising sentiment of support or enthusiasm, especially
      among the general public; as, a groundswell of opinion
      favoring campaign finance reform.
      [PJC]