Definition
An imaginary line running north and south on the surface of the Earth, passing through both the North and South Poles. Meridians are lines of longitude, used together with parallels of latitude to identify any point on the Earth's surface. The Prime Meridian, which passes through Greenwich, England, is designated 0° longitude, and all other meridians are measured east or west from it up to 180°.
Plain English
A meridian is one of the up-and-down lines you see on a globe or chart that runs from the North Pole to the South Pole. It tells you how far east or west a place is.
Context Anchor
Seen on aeronautical charts, navigation discussions, and position descriptions that use latitude and longitude.
Derivation
From the Latin meridianus, meaning 'of midday' or 'noon.' At any location, the sun crosses the local meridian at noon — that is, when the sun is at its highest point in the sky directly to the south or north. This is why these north–south lines are called meridians: they are the lines along which it is noon at the same instant.
Why Pilots Care
Accurate meridian references are essential for determining exact position, adjusting time zones, and maintaining proper navigation during cross-country flight.
Analogy
Think of meridians as the vertical seams on a peeled orange that all meet at the top and bottom.
Intuition Check
A meridian is not a line that runs around the Earth east-west. It runs from pole to pole and is used for longitude, not latitude.
Example Sentence 1
The pilot tracked along the 95° west meridian as she flew south toward the Gulf coast.
Example Sentence 2
Crossing each meridian changes the local time by four minutes when flying east or west.