Nadir

The Poles of the Horizon which indicates its extremities according to cardinal points of the compass, South to North, East to West. (Nadir is directly below the observer, Zenith overhead.) A line drawn between Zenith and Nadir would be in the direction of the gravitational pull, like hanging a pendulum or plumb line. The nadir is in the horizonal coordinate system what the Imum coeli or lower midheaven is to the ecliptic coordinate system; the two cannot be used interchangeably.