prime meridian
Geographical term for the 0° longitude line, specifically the one passing through Greenwich.
本初子午線はイギリスのグリニッジを通っている。
The prime meridian passes through Greenwich, England.
赤道 is the reference line for latitude (0°), whereas 本初子午線 is the reference line for longitude (0°), running perpendicular to it.
Compound of 本初 (ほんしょ, 'origin' or 'beginning') and 子午線 (しごせん, 'meridian'). The term is a direct translation of the English 'prime meridian'.