Astronomy Answers
From the Astronomical Dictionary

The description of the word you requested from the astronomical dictionary is given below.

the meridian

meridianus = [Latin] concering noon

A meridian is

1. a half-circle (semicircle) that runs across the surface between the geographic poles of a planetary body. Meridians intersect the equator at right angles. Observers on the same meridian have the same solar time.
2. the half-circle that runs from the equatorial north pole (near the Pole Star) via the zenith to the equatorial south pole. When the Sun goes through the meridian, then it is highest in the sky for the day, and then it is noon. Also called the celestial meridian.