As a noun, a meridian is an imaginary line that runs from the North Pole to the South Pole on a map or globe, used to measure longitude. In older usage, it can also mean the highest point of something, like the 'meridian of life'.
From Latin “meridianus” meaning 'of midday, southern', from “meridies” 'midday, south', from “medius” 'middle' and “dies” 'day'. The link to north–south lines comes from the sun being due south (in the Northern Hemisphere) at midday. The idea of a 'highest point' grew from the image of the sun at its noon peak.
Meridian started as a word about noon—the sun’s highest, brightest moment—before it became a geography term. On Earth maps, meridians are like slices of orange from pole to pole, all meeting at the 'noon' line somewhere on the planet at any given moment.
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