From Latin 'medius' (middle) + 'dies' (day) — the line the sun crosses at noon, extended to any line of longitude.
An imaginary great circle on the Earth's surface passing through the North and South Poles and any given point; the highest point reached by a celestial body in its apparent daily arc across the sky; figuratively, the peak or zenith of achievement.
From Old French 'meridien,' from Latin 'meridianus' (of midday, pertaining to noon), from 'meridies' (midday, noon, south), altered from older 'medidies,' from 'medius' (middle) + 'dies' (day), from PIE *medhyo- (middle) + *dyew- (sky, heaven, day, god). A meridian is literally the 'midday line' — the line across the sky that the Sun crosses at noon. When the Sun crosses a given meridian, it is noon at that longitude. The geographical sense (a line of longitude) derives from this astronomical definition. Key roots
A 'meridian' literally means 'midday-line' — from Latin 'medius' (middle) + 'dies' (day). The same 'dies' gives us 'diary' (a daily record), 'dial' (a daily timekeeper), and 'Jupiter' (from 'Diu-pater,' Sky-Father). Noon defines the meridian because that is when the Sun is at its highest point