The Collaborative International Dictionary of English v.0.48:

Zenith \Ze"nith\ (?; 277), n. [OE. senyth, OF. cenith, F.
   z['e]nith, Sp. zenit, cenit, abbrev. fr. Ar. samt-urras way
   of the head, vertical place; samt way, path + al the + ras
   head. Cf. Azimuth.]
   [1913 Webster]
   1. That point in the visible celestial hemisphere which is
      vertical to the spectator; the point of the heavens
      directly overhead; -- opposed to nadir.
      [1913 Webster]

            From morn
            To noon he fell, from noon to dewy eve,
            A summer's day; and with the setting sun
            Dropped from the zenith, like a falling star.
                                                  --Milton.
      [1913 Webster]

   2. hence, figuratively, the point of culmination; the
      greatest height; the height of success or prosperity.
      [1913 Webster]

            I find my zenith doth depend upon
            A most auspicious star.               --Shak.
      [1913 Webster]

            This dead of midnight is the noon of thought,
            And wisdom mounts her zenith with the stars. --Mrs.
                                                  Barbauld.
      [1913 Webster]

            It was during those civil troubles . . . this
            aspiring family reached the zenith.   --Macaulay.
      [1913 Webster]

   Zenith distance. (Astron.) See under Distance.

   Zenith sector. (Astron.) See Sector, 3.

   Zenith telescope (Geodesy), a telescope specially designed
      for determining the latitude by means of any two stars
      which pass the meridian about the same time, and at nearly
      equal distances from the zenith, but on opposite sides of
      it. It turns both on a vertical and a horizontal axis, is
      provided with a graduated vertical semicircle, and a level
      for setting it to a given zenith distance, and with a
      micrometer for measuring the difference of the zenith
      distances of the two stars.
      [1913 Webster]

WordNet (r) 3.0 (2006):

zenith
    n 1: the point above the observer that is directly opposite the
         nadir on the imaginary sphere against which celestial
         bodies appear to be projected [ant: nadir]

Moby Thesaurus II by Grady Ward, 1.0:

102 Moby Thesaurus words for "zenith":
   Olympian heights, acme, aerial heights, apex, apogee, authority,
   authorization, be-all and end-all, blue ribbon, brow, cap,
   capstone, championship, climax, cloud nine, command, control,
   crest, crown, culmen, culmination, directorship, dizzy heights,
   dominion, edge, effectiveness, elevation, eminence, ether,
   extreme limit, extremity, first place, first prize, headship,
   heaven, heavens, hegemony, height, heights, high noon, high point,
   highest, highest pitch, highest point, imperium, influence,
   jurisdiction, kingship, leadership, lift, limit, lordship,
   management, mastership, mastery, maximum, meridian, most,
   mountaintop, ne plus ultra, new high, no place higher, noon, palms,
   paramountcy, peak, pinnacle, pitch, point, pole, power, presidency,
   primacy, raise, record, ridge, rise, rising ground, rule, say,
   seventh heaven, sky, sovereignty, spire, steep, stratosphere,
   summit, supremacy, sway, tip, tip-top, top, top spot, upmost,
   upper extremity, uppermost, uprise, utmost, vantage ground,
   vantage point, vertex, very top



The Devil's Dictionary (1881-1906):

ZENITH, n.  The point in the heavens directly overhead to a man
standing or a growing cabbage.  A man in bed or a cabbage in the pot
is not considered as having a zenith, though from this view of the
matter there was once a considerably dissent among the learned, some
holding that the posture of the body was immaterial.  These were
called Horizontalists, their opponents, Verticalists.  The
Horizontalist heresy was finally extinguished by Xanobus, the
philosopher-king of Abara, a zealous Verticalist.  Entering an
assembly of philosophers who were debating the matter, he cast a
severed human head at the feet of his opponents and asked them to
determine its zenith, explaining that its body was hanging by the
heels outside.  Observing that it was the head of their leader, the
Horizontalists hastened to profess themselves converted to whatever
opinion the Crown might be pleased to hold, and Horizontalism took its
place among _fides defuncti_.