In the Sky
In the sky, a meridian is an imaginary great circle on the celestial sphere that is perpendicular to the local horizon. It passes through the north point on the horizon, through the celestial pole, up to the zenith, and through the south point on the horizon.
Because it is fixed to the local horizon, stars will appear to drift past the local meridian as the earth spins. You can use an object's right ascension and the local sidereal time to determine when it will cross your local meridian, or culminate (see hour angle).
The upper meridian is the half above the horizon, the lower meridian the half below it.
On the Earth
On the earth, a meridian is a 'straight line' on the earth's surface between the North Pole and the South Pole (in fact, half of a great circle). Considering the meridian that passes through Greenwich, England, as establishing the meaning of zero degrees of longitude, or the prime meridian, any other meridian is identified by the angle, referenced to the center of the earth as vertex, between where it and the prime meridian cross the equator. As there are 360 degrees in a circle, the meridian on the opposite side of the earth from Greenwich (which forms the other half of a circle with the one through Greenwich) is 180° longitude, and the others lie between 0° and 180° of West longitude in the Western Hemisphere (west of Greenwich) and between 0° and 180° of East longitude in the Eastern Hemisphere (east of Greenwich). You can see the lines of longitude on most maps.
The term "meridian" comes from the Latin meridies, meaning "midday"; the sun crosses a given meridian midway between the times of sunrise and sunset on that meridian.
This article originates from Jason Harris' Astroinfo which comes along with KStars, a Desktop Planetarium for Linux/KDE. See http://edu.kde.org/kstars/index.phtml
Meridian is also the name of some places in the United States of America:
Meridian is also a novel by Alice Walker.