ascendant (adj.)

late 14c., ascendent, in astrology, "rising over the horizon," from Latin ascendentem (nominative ascendans), present participle of ascendere "to mount, ascend, go up" (see ascend). Sense "moving upward, rising" is recorded from 1590s.

As a noun in astrology, "point of the ecliptic or sign of the zodiac which is on the eastern horizon at the moment of birth." The planet that rules the ascendant is believed to have predominant influence on the horoscope. Hence in the ascendant "ruling, dominant" (not, as is often thought, "rising"), 1670s, and the adjective meaning "superior, dominant," 1806.

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Definitions of ascendant from WordNet
ascendant (n.)
position or state of being dominant or in control;
that idea was in the ascendant
Synonyms: ascendent
ascendant (n.)
someone from whom you are descended (but usually more remote than a grandparent);
Synonyms: ancestor / ascendent / antecedent / root
ascendant (adj.)
tending or directed upward; "rooted and ascendant strength like that of foliage"- John Ruskin;
Synonyms: ascendent / ascensive
ascendant (adj.)
most powerful or important or influential;
the economically ascendant class
Synonyms: ascendent / dominating