Definition
Imaginary lines on the Earth's surface that run from the magnetic north pole to the magnetic south pole, following the direction of the Earth's magnetic field. A magnetic compass aligns itself along these lines rather than along the lines of true (geographic) north and south.
Plain English
These are the invisible lines a compass needle actually points along. They lead to the magnetic poles, not the true poles at the top and bottom of the globe.
Context Anchor
Seen in navigation and chart discussions of variation, where pilots compare magnetic north with true north.
Derivation
Meridian comes from the Latin meridianus, meaning 'midday' or 'noon line.' Originally it described the imaginary line connecting points where the sun is directly overhead at noon, which runs north–south. Magnetic meridians borrow the same idea of north–south lines, but follow the Earth's magnetic field instead of its geographic axis.
Why Pilots Care
The difference between magnetic and true meridians creates variation that must be corrected to fly accurate headings.
Analogy
Think of magnetic meridians like invisible lanes that a compass needle wants to line up with. Those lanes are close to true north-south in many places, but they are not exactly the same.
Intuition Check
Do not assume a magnetic meridian is the same as a longitude line on a map. A longitude line is based on true north; a magnetic meridian is based on where a compass points.
Example Sentence 1
Because magnetic meridians do not line up with true meridians, pilots apply variation when converting between true and magnetic headings.
Example Sentence 2
Isogonic lines on the sectional chart show how much the magnetic meridians deviate from true meridians at different locations.