substructure
     n 1: the basic structure or features of a system or organization
          [syn: {infrastructure}]
     2: lowest support of a structure; "it was built on a base of
        solid rock"; "he stood at the foot of the tower" [syn: {foundation},
         {base}, {fundament}, {foot}, {groundwork}, {understructure}]
Source: WordNet® 2.0