means An imaginary great circle or line running from pole to pole, used to mark longitude and divide the day — or, more broadly, the highest point something reaches.

from From Latin meridianus, 'of midday,' from meridies, 'noon' — itself a worn-down form of medidies, 'mid-day' (medius, 'middle,' plus dies, 'day'). The sense of 'noon' carried over because the sun crosses the local meridian line exactly at midday; from that overhead peak comes the figurative 'meridian' meaning the height of one's powers or fortunes.