The Collaborative International Dictionary of English v.0.48:

Ascendant \As*cend"ant\, Ascendent \As*cend"ent\, a.
   1. Rising toward the zenith; above the horizon.
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            The constellation . . . about that time ascendant.
                                                  --Browne.
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   2. Rising; ascending. --Ruskin.
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   3. Superior; surpassing; ruling.
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            An ascendant spirit over him.         --South.
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            The ascendant community obtained a surplus of
            wealth.                               --J. S. Mill.
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            Without some power of persuading or confuting, of
            defending himself against accusations, . . . no man
            could possibly hold an ascendent position. --Grote.
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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

WordNet (r) 3.0 (2006):

ascendant
    adj 1: tending or directed upward; "rooted and ascendant
           strength like that of foliage"- John Ruskin [syn:
           ascendant, ascendent, ascensive]
    2: most powerful or important or influential; "the economically
       ascendant class"; "D-day is considered the dominating event
       of the war in Europe" [syn: ascendant, ascendent,
       dominating]
    n 1: position or state of being dominant or in control; "that
         idea was in the ascendant" [syn: ascendant, ascendent]
    2: someone from whom you are descended (but usually more remote
       than a grandparent) [syn: ancestor, ascendant,
       ascendent, antecedent, root] [ant: descendant,
       descendent]