Plain English Meaning
The vertical lines you see on a globe or chart that run from the top of the Earth to the bottom. They are used to describe how far east or west a place is.
Full Definition
Imaginary north–south lines drawn on the Earth's surface that connect the geographic North Pole to the geographic South Pole, used to measure angular distance east or west of the Prime Meridian (0°) running through Greenwich, England. Meridians are numbered from 0° to 180° east and 0° to 180° west.
Context Anchor
Seen in magnetic compass error discussions when comparing true north, based on the Earth’s geographic poles, with magnetic north, based on the Earth’s magnetic field.
Derivation
From the Latin meridianus, meaning 'of midday,' because the sun crosses a given at local noon. '' comes from the Latin longitudo, meaning 'length' — the long lines running the length of the globe from pole to pole.