Source : Webster's Revised Unabridged Dictionary (1913)

Meridian \Me*rid"i*an\, n. [F. m['e]ridien. See {Meridian}, a.]
   1. Midday; noon.

   2. Hence: The highest point, as of success, prosperity, or
      the like; culmination.

            I have touched the highest point of all my
            greatness, And from that full meridian of my glory I
            haste now to my setting.              --Shak.

   3. (Astron.) A great circle of the sphere passing through the
      poles of the heavens and the zenith of a given place. It
      is crossed by the sun at midday.

   4. (Geog.) A great circle on the surface of the earth,
      passing through the poles and any given place; also, the
      half of such a circle included between the poles.

   Note: The planes of the geographical and astronomical
         meridians coincide. Meridians, on a map or globe, are
         lines drawn at certain intervals due north and south,
         or in the direction of the poles.

   {Calculated for}, or {fitted to}, or {adapted to}, {the
   meridian of}, suited to the local circumstances,
      capabilities, or special requirements of.

            All other knowledge merely serves the concerns of
            this life, and is fitted to the meridian thereof.
                                                  --Sir M. Hale.

   {First meridian}, the meridian from which longitudes are
      reckoned. The meridian of Greenwich is the one commonly
      employed in calculations of longitude by geographers, and
      in actual practice, although in various countries other
      and different meridians, chiefly those which pass through
      the capitals of the countries, are occasionally used; as,
      in France, the meridian of Paris; in the United States,
      the meridian of Washington, etc.

   {Guide meridian} (Public Land Survey), a line, marked by
      monuments, running North and South through a section of
      country between other more carefully established meridians
      called principal meridians, used for reference in
      surveying. [U.S.]

   {Magnetic meridian}, a great circle, passing through the
      zenith and coinciding in direction with the magnetic
      needle, or a line on the earth's surface having the same
      direction.

   {Meridian circle} (Astron.), an instrument consisting of a
      telescope attached to a large graduated circle and so
      mounted that the telescope revolves like the transit
      instrument in a meridian plane. By it the right ascension
      and the declination of a star may be measured in a single
      observation.

   {Meridian instrument} (Astron.), any astronomical instrument
      having a telescope that rotates in a meridian plane.

   {Meridian of a globe}, or {Brass meridian}, a graduated
      circular ring of brass, in which the artificial globe is
      suspended and revolves.

Source : WordNet®

magnetic meridian
     n : an imaginary line passing through both magnetic poles of the
         Earth