an imiginary line denoting o degrees longitude
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Greenwich is an observatory in London England, assigned as the "starting point" for the time zones.
There is only one Greenwich Meridian (which I think is what you're asking about). It is the zero degrees line of longitude, so when a map location is given the longitude is given as being east or west of Greenwich. It position, going through Greenwich is partly historical and partly practical - opposite the meridian is the International Date Line, which for practical reasons is placed in the sea.
Greenwich England
GMT=Greenwich Mean Time
Since the early 20th Century, the Prime Meridian has been agreed by all nations to be the meridian that passes through Greenwich, England. It is sometimes called the Greenwich Meridian.