meridian

English meridian comes from Late Middle English meridian/meridien, via Anglo-Norman and Middle French meridien, from Latin merīdiānum, the neuter noun form of merīdiānus ("relating to midday; southern"). That adjective traces to merīdiēs ("midday, noon"), a dissimilated form of Old Latin medīdiēs, itself built from medius ("middle," from PIE *médʰyos) plus diēs ("day," from PIE *dyew-, "heaven, sky").

Originally tied to the literal middle of the day — noon, and the sun's position then — the word expanded to mean 'the south' (since the sun sits southward at noon in the Northern Hemisphere), then came to name imaginary lines of longitude and the celestial meridian in astronomy. Later, English also borrowed a separate sense from French méridien and German Meridian describing the body's energy pathways, itself a calque of Mandarin jīng.

Meridian shares its root with medius, the Latin word for 'middle,' also behind English words like 'medium' and 'median.'