Source : Webster's Revised Unabridged Dictionary (1913)

Frontier \Fron"tier\, v. i.
   To constitute or form a frontier; to have a frontier; -- with
   on. [Obs.] --Sir W. Temple.

Frontier \Fron"tier\, n. [F. fronti[`e]re, LL. frontaria. See
   {Front}.]
   1. That part of a country which fronts or faces another
      country or an unsettled region; the marches; the border,
      confine, or extreme part of a country, bordering on
      another country; the border of the settled and cultivated
      part of a country; as, the frontier of civilization.

   2. (Fort.) An outwork. [Obs.]

            Palisadoes, frontiers, parapets.      --Shak.

Frontier \Fron"tier\, a.
   1. Lying on the exterior part; bordering; conterminous; as, a
      frontier town.

   2. Of or relating to a frontier. ``Frontier experience.''
      --W. Irving.

Source : WordNet®

frontier
     n 1: a wilderness at the edge of a settled area of a country;
          "the individualism of the frontier in Andrew Jackson's
          day"
     2: an international boundary or the area (often fortified)
        immediately inside the boundary
     3: an undeveloped field of study; a topic inviting research and
        development; "he worked at the frontier of brain science"