Meridian
late Middle English: from Old French meridien, from Latin meridianum (neuter, used as a noun) ‘noon’, from medius ‘middle’ + dies ‘day’. The use in astronomy is due to the fact that the sun crosses a meridian at noon.
wiktionary
Middle English meridian, from Anglo-Norman meridien, Middle French meridien(“midday; the south; celestial meridian”), and their source, Latin merīdiānum, noun use of neuter form of merīdiānus(“meridian”), ultimately from medius(“middle”) + diēs(“day”).
etymonline
meridian (n.)
mid-14c., "noon, midday," from Old French meridien "of the noon time, midday; the meridian; a southerner" (12c.), and directly from Latin meridianus "of midday, of noon, southerly, to the south," from meridies "noon, south," from meridie "at noon," altered by dissimilation from pre-Latin *medi die, locative of medius "mid-" (from PIE root *medhyo- "middle") + dies "day" (from PIE root *dyeu- "to shine").
The cartographic sense of "a great circle or half-circle of a sphere passing through the poles" is attested from late 14c., originally astronomical. Figurative uses tend to suggest "point of highest development or fullest power," implying a subsequent decline. As an adjective from late 14c. Related: Meridional. The city in Mississippi, U.S., was settled 1854 (as Sowashee Station) at a railway junction and given its current name in 1860, supposedly by people who thought meridian meant "junction" (they perhaps confused the word with median).
