the prime meridian
noun [ S ] geography specialized uk/ˌpraɪm məˈrɪd.i.ən/ us/ˌpraɪm məˈrɪd.i.ən/
本初子午线（从北机到南极的一根假想线，它穿过英格兰的格林尼治，标示为经线0°，其他经线以此为测定标准）
the imaginary line from the North Pole to the South Pole that passes through Greenwich in England and marks 0° longitude, from which all other longitudes are measured