/
ˈneɪdɪə, 
ˈnadɪə/
n.
  - the lowest or most unsuccessful point:
 his fortunes reached their nadir.
 - Astronomy  the point on the celestial sphere directly below an observer. The opposite of zenith.
 
Etymology
ME: via Fr. from Arab. naẓīr (as-samt) ‘opposite (to the zenith)’.