Magnetic meridian is an imaginary:

The magnetic meridian is defined as an imaginary vertical plane passing through the magnetic axis of a freely suspended magnetic needle [2]. While it is often simplified as a line on the Earth's surface that connects the magnetic north and south poles, in the context of geomagnetism and physics, it is specifically the vertical plane that contains the magnetic field vector at a given point [2]. This plane is used to measure magnetic declination, which is the angle between the geographic meridian (the vertical plane through the geographic poles) and the magnetic meridian [2]. A compass needle aligns itself within this vertical plane, pointing toward the magnetic north [1]. Therefore, while it manifests as a line of direction on a map, its formal geometric definition in terrestrial magnetism is a vertical plane.

Sources

1. [1] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 5: Earths Magnetic Field (Geomagnetic Field) > Magnetic Inclination or Magnetic Dip or Dip Angle > p. 77
2. [2] https://www.khanacademy.org/science/in-in-class-12th-physics-india/x51bd77206da864f3:magnetism-matter/x51bd77206da864f3:earth-s-magnetic-field-magnetic-elements/v/magnetic-declination-earth-s-magnetism