DuckCorp Dico

Found one definition

1.                 From WordNet (r) 3.0 (2006):
                   
   
   
                   
                     
   sill
       n 1: structural member consisting of a continuous horizontal
            timber forming the lowest member of a framework or
            supporting structure
       2: (geology) a flat (usually horizontal) mass of igneous rock
          between two layers of older sedimentary rock