Noah Webster's New International Dictionary of the English Language

1. (a.) Being at, or pertaining to, midday; belonging to, or passing through, the highest point attained by the sun in his diurnal course.

2. (a.) Pertaining to the highest point or culmination; as, meridian splendor.

3. (a.) Midday; noon.

4. (a.) Hence: The highest point, as of success, prosperity, or the like; culmination.

5. (n.) 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.

6. (n.) 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.