The Collaborative International Dictionary of English v.0.48:

Prime \Prime\, a. [F., fr. L. primus first, a superl.
   corresponding to the compar. prior former. See Prior, a.,
   Foremost, Former, and cf. Prim, a., Primary,
   Prince.]
   1. First in order of time; original; primeval; primitive;
      primary. "Prime forests." --Tennyson.
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            She was not the prime cause, but I myself. --Milton.
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   Note: In this sense the word is nearly superseded by
         primitive, except in the phrase prime cost.
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   2. First in rank, degree, dignity, authority, or importance;
      as, prime minister. "Prime virtues." --Dryden.
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   3. First in excellence; of highest quality; as, prime wheat;
      a prime quality of cloth.
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   4. Early; blooming; being in the first stage. [Poetic]
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            His starry helm, unbuckled, showed him prime
            In manhood where youth ended.         --Milton.
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   5. Lecherous; lustful; lewd. [Obs.] --Shak.
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   6. Marked or distinguished by a mark (') called a prime mark.

   Note: In this dictionary the same typographic mark is used to
         indicate a weak accent in headwords, and minutes of a
         degree in angle measurements.
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   7. (Math.)
      (a) Divisible by no number except itself or unity; as, 7
          is a prime number.
      (b) Having no common factor; -- used with to; as, 12 is
          prime to 25.
          [Webster 1913 Suppl.]

   Prime and ultimate ratio. (Math.). See Ultimate.

   Prime conductor. (Elec.) See under Conductor.

   Prime factor (Arith.), a factor which is a prime number.

   Prime figure (Geom.), a figure which can not be divided
      into any other figure more simple than itself, as a
      triangle, a pyramid, etc.

   Prime meridian (Astron.), the meridian from which longitude
      is reckoned, as the meridian of Greenwich or Washington.


   Prime minister, the responsible head of a ministry or
      executive government; applied particularly to that of
      England.

   Prime mover. (Mech.)
      (a) A natural agency applied by man to the production of
          power. Especially: Muscular force; the weight and
          motion of fluids, as water and air; heat obtained by
          chemical combination, and applied to produce changes
          in the volume and pressure of steam, air, or other
          fluids; and electricity, obtained by chemical action,
          and applied to produce alternation of magnetic force.
      (b) An engine, or machine, the object of which is to
          receive and modify force and motion as supplied by
          some natural source, and apply them to drive other
          machines; as a water wheel, a water-pressure engine, a
          steam engine, a hot-air engine, etc.
      (c) Fig.: The original or the most effective force in any
          undertaking or work; as, Clarkson was the prime mover
          in English antislavery agitation.

   Prime number (Arith.), a number which is exactly divisible
      by no number except itself or unity, as 5, 7, 11.

   Prime vertical (Astron.), the vertical circle which passes
      through the east and west points of the horizon.

   Prime-vertical dial, a dial in which the shadow is
      projected on the plane of the prime vertical.

   Prime-vertical transit instrument, a transit instrument the
      telescope of which revolves in the plane of the prime
      vertical, -- used for observing the transit of stars over
      this circle.
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The Collaborative International Dictionary of English v.0.48:

Meridian \Me*rid"i*an\, n. [F. m['e]ridien. See Meridian, a.]
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   1. Midday; noon.
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   2. Hence: The highest point, as of success, prosperity, or
      the like; culmination.
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            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.
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   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.
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   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.
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   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.
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   Calculated for the meridian of, or fitted to the meridian
   of, or adapted to the meridian of, suited to the local
      circumstances, capabilities, or special requirements of.
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            All other knowledge merely serves the concerns of
            this life, and is fitted to the meridian thereof.
                                                  --Sir M. Hale.
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   First meridian or prime 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.
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WordNet (r) 3.0 (2006):

prime meridian
    n 1: meridian at zero degree longitude from which east and west
         are reckoned (usually the Greenwich longitude in England)