Nadir
Etymology
From Arabic naẓir as-samt, "opposite the zenith," literally the point on the celestial sphere directly beneath the observer, straight down through the earth. It entered European languages through medieval Arabic astronomy alongside "zenith" and "azimuth." From that precise astronomical low point came the figurative sense of any low ebb.
noun · /NAY-deer/
The lowest point of something — the bottom of a decline or fortune.
What it names
The absolute low, the moment or level from which things can only hold or improve; used for careers, moods, markets, and reputations.
Example
The band’s nadir came when they played to a half-empty bar the same year they had once headlined stadiums.
Related idea
Zenith is its literal opposite, the highest point; where zenith marks the peak overhead, nadir marks the point directly below.
noun · /NAY-deer/
In astronomy, the point on the celestial sphere directly beneath an observer, straight down through the earth.
What it names
the literal opposite of the zenith — the downward pole of one’s local sky
Example
The sun passes through the observer’s nadir at local midnight.
Related idea
The figurative low point descends from this: the nadir of one’s fortunes borrows the astronomical bottom of the sky as metaphor. Its opposite overhead is the zenith.