Line at zero degrees longitude (5,8)

I believe the answer is:

prime meridian

'line at zero degrees longitude' is the definition.
(I've seen this in another clue)

This is the entire clue.

(Other definitions for prime meridian that I've seen before include "Line of 0 longitude" , "line through London" .)

I've seen this clue in the USA Today and the LA Times.

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