Definition
The meridian of zero degrees longitude, running from the North Pole to the South Pole through Greenwich, England, used as the reference line from which all other longitudes east and west are measured.
Plain English
It is the line on a globe marked 0° longitude. Every other line of longitude is measured as so many degrees east or west of it.
Context Anchor
Seen in latitude and longitude discussions, navigation charts, and position coordinates.
Derivation
From Latin primus, meaning 'first,' and meridianus, meaning 'of midday' (the line the sun crosses at noon). So 'prime meridian' literally means the 'first noon line' — the chosen starting line from which all other longitude lines are counted.
Why Pilots Care
It provides the fixed reference needed to calculate exact longitude positions for route planning and position reporting.
Analogy
The Prime Meridian is like the zero mark on a ruler. Once you know where zero is, you can measure distance in either direction from it.
Intuition Check
Prime does not mean “best” here. It means the chosen zero line used as the reference for measuring longitude.
Example Sentence 1
The pilot noted that the airport's longitude was about 77 degrees west of the prime meridian.
Example Sentence 2
All longitudes on the sectional chart are measured from the Prime Meridian.