1. That point in the visible celestial hemisphere which is vertical to the spectator; the point of the heavens directly overhead; opposed to nadir. 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)
2. Hence, figuratively, the point of culmination; the greatest height; the height of success or prosperity. I find my zenith doth depend upon A most auspicious star. (Shak) This dead of midnight is the noon of thought, And wisdom mounts her zenith with the stars. (Mrs. Barbauld) It was during those civil troubles . . . This aspiring family reached the zenith. (Macaulay) zenith distance.
(Science: astronomy) 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.