The Collaborative International Dictionary of English v.0.48:

Ascendant \As*cend"ant\, n. [F. ascendant, L. ascendens; p. pr.
   of ascendere.]
   1. Ascent; height; elevation. [R.]
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            Sciences that were then in their highest ascendant.
                                                  --Temple.
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   2. (Astrol.) The horoscope, or that degree of the ecliptic
      which rises above the horizon at the moment of one's
      birth; supposed to have a commanding influence on a
      person's life and fortune.
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   Note: Hence the phrases

   To be in the ascendant, to have commanding power or
      influence, and

   Lord of the ascendant, one who has possession of such power
      or influence; as, to rule, for a while, lord of the
      ascendant. --Burke.
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   3. Superiority, or commanding influence; ascendency; as, one
      man has the ascendant over another.
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            Chievres had acquired over the mind of the young
            monarch the ascendant not only of a tutor, but of a
            parent.                               --Robertson.
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   4. An ancestor, or one who precedes in genealogy or degrees
      of kindred; a relative in the ascending line; a
      progenitor; -- opposed to descendant. --Ayliffe.
      [1913 Webster] Ascendant