Ascent means (1) the act of climbing or going upward, (2) advancement, and (3) an incline. Ascendance is a mainly American variant of ascendancy, which means the state of having dominance. Ascendence and ascendency—with an e in place of the second a—are also listed in dictionaries as variants of ascendancy, but both are rare in published writing.
In U.S. publications, ascendance and ascendancy appear about equally often, while ascendance is almost nowhere to be found in publications from outside the U.S. In either form, the word derives from the phrase in the ascendant, which means not in a state of ascent but rather in a state of dominance. The weirdness in meaning is due to ascendant‘s idiosyncratic astrological senses.
Ascendance and ascendancy are so often used in place of ascent that we may have to accept that their meaning has changed. For now, though, we can still find plenty of examples of the words used correctly in their traditional sense.
Most English speakers are comfortable with ascent, so we won’t provide examples for that word. Instead, let’s look at a few examples of ascendance and ascendancy used well. These are from American publications:
With Christian conservatives in the ascendancy in the Republican Party, Islam-bashing may still have a future. [Los Angeles Times]
Jobs’s absence from Apple coincided with the ascendance of Bill Gates and Microsoft Corp., developer of a graphics-driven operating system of its own called Windows. [Dallas Morning News]
These are from outside the U.S.:
For a while, it was wholly unclear which side would win, and indeed for long periods it appeared that the Left was in the ascendancy. [Telegraph]
This was the era of absolute ascendancy for Joh Bjelke-Petersen and the National Party. [Daily Telegraph]
In July Michele Bachmann, a shrill congresswoman, had a moment of ascendancy. [Economist]
And in the following instances, ascendance/ascendancy would bear replacement with ascent:
The ascendance through Dante’s mountain of Purgatory is made via a large, spiral staircase. [New York Times]
In his memoir, Cain looks back to growing under segregation in the South and ahead to his hoped for ascendance to the White House. [Washington Post]
That such examples are easily found in normally well-edited publications suggests that ascendance and ascendancy are now accepted variants of ascent.