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]

Moby Thesaurus II by Grady Ward, 1.0:

152 Moby Thesaurus words for "ascendant":
   a cut above, above, absolute, ahead, anabatic, antecedent,
   ascendancy, ascending, ascensional, ascensive, at the head,
   authoritarian, authoritative, authorized, autocratic, better, boss,
   capping, chief, chosen, climbing, clothed with authority,
   commanding, competent, conquering, consequential, considerable,
   controlling, defeating, distinguished, dominance, dominant,
   domination, dominion, duly constituted, eclipsing, eminent,
   empowered, ex officio, exceeding, excellent, excelling, finer,
   flushed with success, forebear, forefather, forerunner, general,
   governing, great, greater, head, hegemonic, hegemonistic, higher,
   imperative, important, in ascendancy, in charge, in chief,
   in the ascendant, influential, leading, leaping, major, marked,
   master, masterdom, mighty, momentous, monocratic, mounting,
   of choice, official, on the throne, one up on, outstanding, over,
   overbearing, overcoming, paramount, potent, powerful, precursor,
   predecessor, predominant, predominate, preeminence, preeminent,
   prepollent, preponderance, preponderant, preponderate, prepotence,
   prepotent, prestigious, prevailing, prevalent, primogenitor,
   progenitor, prominent, puissant, rampant, ranking, rare, rearing,
   regnant, regulating, regulative, regulatory, reigning, rising,
   rivaling, ruling, saltatory, scandent, scansorial, senior,
   skyrocketing, sovereign, sovereignty, spiraling, springing,
   substantial, successful, super, superior, supreme, surpassing,
   swaying, topping, totalitarian, transcendent, transcendental,
   transcending, triumphal, triumphant, uparching, upcoming, upgoing,
   upgrade, uphill, uphillward, upper, uprising, upsloping, upward,
   upwith, vanquishing, victorious, weighty, winning