It’s measured along a meridian, in the main (8,4)
I believe the answer is:
nautical mile
'it's measured along a meridian' is the definition.
(nautical unit of distance)
I don't understand the rest of the clue.
(Other definitions for nautical mile that I've seen before include "Marine distance" , "Unit of distance" , "Distance of approximately 2,000 yards at sea" , "Unit of length, about 6,082 feet" , "1,852 metres" .)