A Meridian is a circle passing around the Earth, through both poles. It is an example of a Great Circle, a circle passing fully around the Earth as the equator does, and a meridian could also be described as a great circle passing through a pole. At any given point on Earth between the poles, only one meridian passes through. The Prime Meridian (aka Greenwich Meridian), passing through Greenwich, England, is used as a reference for maps, Earth coordinates, and standardized time.
Ecliptic Coordinate System
Equatorial Coordinate System