Astronomy Answers
From the Astronomical Dictionary

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The description of the word you requested from the astronomical dictionary is given below.

the prime meridian

plural: prime meridians

The prime meridian is the meridian of which the longitude is zero. It is a free choice which meridian should be the prime meridian. In the past, each country used to use a prime meridian for their maps that ran through their own capital city. Nowadays almost everyone uses the prime meridian that runs through Greenwich in England.