a. [ L. antemeridianus; ante + meridianus belonging to midday or noon. See Meridian. ] Being before noon; in or pertaining to the forenoon. (Abbrev. a. m.) [ 1913 Webster ]

a. [ Pref. circum- + meridian. ] About, or near, the meridian. [ 1913 Webster ]

(Physics), n. An imaginary line passing through both magnetic poles of the earth. [ WordNet 1.5 ]

a. [ F. méridien, L. meridianus pertaining to noon, fr. meridies noon, midday, for older medidies; medius mid, middle + dies day. See Mid, and Diurnal. ] 1. Being at, or pertaining to, midday; belonging to, or passing through, the highest point attained by the sun in his diurnal course. “Meridian hour.” Milton. [ 1913 Webster ]

Tables . . . to find the altitude meridian. Chaucer. [ 1913 Webster ]

2. Pertaining to the highest point or culmination; as, meridian splendor. [ 1913 Webster ]

n. [ F. méridien. See Meridian, a. ] [ 1913 Webster ]

1. Midday; noon. [ 1913 Webster ]

2. Hence: The highest point, as of success, prosperity, or the like; culmination. [ 1913 Webster ]

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. [ 1913 Webster ]

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. [ 1913 Webster ]

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. [ 1913 Webster ]

☞ 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. [ 1913 Webster ]

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. [ 1913 Webster ]

All other knowledge merely serves the concerns of this life, and is fitted to the meridian thereof. Sir M. Hale. [ 1913 Webster ]

--
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. [ 1913 Webster ]

a. [ L. postmeridianus; post after + meridianus. See Meridian. ] 1. Coming after the sun has passed the meridian; being in, or belonging to, the afternoon. (Abbrev. P. M.) [ 1913 Webster ]

2. Fig., belonging to the after portion of life; late. [ R. ] [ 1913 Webster ]