1.  nadir(n = noun.state) low-water mark - an extreme state of adversity; the lowest point of anything;
is a kind of adversity, hard knocks, hardship

2.  nadir(n = noun.location) - the point below the observer that is directly opposite the zenith on the imaginary sphere against which celestial bodies appear to be projected;
is a kind of celestial point
is a part of celestial sphere, empyrean, firmament, heavens, sphere, vault of heaven, welkin
Antonym: zenith

1.  That point of the heavens, or lower hemisphere, directly opposite the zenith; the inferior pole of the horizon; the point of the celestial sphere directly under the place where we stand. [1913 Webster]

2.  The lowest point; the time of greatest depression. [1913 Webster]
"The seventh century is the nadir of the human mind in Europe." [1913 Webster]

nadir, n.
1 the part of the celestial sphere directly below an observer (opp. ZENITH).
2 the lowest point in one's fortunes; a time of deep despair.

Etymology:
ME f. OF f. Arab. nazir (as-samt) opposite (to the zenith)

N  base, basement, plinth, dado, wainscot, baseboard, mopboard, bedrock, hardpan, foundation, substructure, substratum, ground, earth, pavement, floor, paving, flag, carped, ground floor, deck, footing, ground work, basis, hold, bilge, bottom, nadir, foot, sole, toe, hoof, keel, root, centerboard, bottom, undermost, nethermost, fundamental, founded on, based on, grounded on, built on.