An imaginary great circle on the earth's surface passing through the North and South geographic poles. All points on the same meridian have the same longitude. Either half of such a great circle from pole to pole. Astronomy A great circle passing through the two poles of the celestial sphere and the zenith of a given observer. Mathematics A curve on a surface of revolution, formed by the intersection of the surface with a plane containing the axis of revolution. Mathematics A plane section of a surface of revolution containing the axis of revolution. Any of the longitudinal lines or pathways on the body along which the acupuncture points are distributed. Archaic The highest point in the sky reached by the sun or another celestial body; a zenith. Archaic Noon. The highest point or stage of development; peak: "Men come to their meridian at various periods of their lives\u201d ( John Henry Newman). Midwestern U.S. See median strip. See Regional Note at neutral ground. Of or relating to a meridian; meridional. Of or at midday: the meridian hour. Of, relating to, or constituting the highest point, as of development or power: the empire in its meridian period.