The Collaborative International Dictionary of English v.0.48:

Interior \In*te"ri*or\, n.
   1. That which is within; the internal or inner part of a
      thing; the inside.
      [1913 Webster]

   2. The inland part of a country, state, or kingdom.
      [1913 Webster]

   Department of the Interior, that department of the
      government of the United States which has charge of
      pensions, patents, public lands and surveys, the Indians,
      education, etc.; that department of the government of a
      country which is specially charged with the internal
      affairs of that country; the home department.

   Secretary of the Interior, the cabinet officer who, in the
      United States, is at the head of the Department of the
      Interior.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Department of the Interior
    n 1: the United States federal department charged with
         conservation and the development of natural resources;
         created in 1849 [syn: Department of the Interior,
         Interior Department, Interior, DoI]