Me·rid·i·an, n.
 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 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.
    All other knowledge merely serves the concerns of this life, and is fitted to the meridian thereof.   --Sir M. Hale.
 -- 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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 Prime a.
 1. First in order of time; original; primeval; primitive; primary. “Prime forests.”
    She was not the prime cause, but I myself.   --Milton.
 Note: ☞ In this sense the word is nearly superseded by primitive, except in the phrase prime cost.
 2. First in rank, degree, dignity, authority, or importance; as, prime minister. “Prime virtues.”
 3. First in excellence; of highest quality; as, prime wheat; a prime quality of cloth.
 4. Early; blooming; being in the first stage. [Poetic]
 His starry helm, unbuckled, showed him prime
 In manhood where youth ended.   --Milton.
 5. Lecherous; lustful; lewd. [Obs.]
 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.
 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.
 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.


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