c. 1300, "ascent of Christ from earth into Heaven in the presence of his disciples on the 40th day after the Resurrection," from Latin ascensionem (nominative ascensio) "a rising," noun of action from past-participle stem of ascendere "to mount, ascend, go up" (see ascend). It is commemorated in the Church as Holy Thursday. Astronomical sense is recorded late 14c.; meaning "action of ascending" is from 1590s. Related: Ascensional.
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.