There is no such math term. There is, on the other hand, an astronomical term "equinox".

aequus (equal) + nox (night)

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The autumnal equinox is also known as the fall equinox and the September equinox. An example of a sentence using "autumnal equinox" is "This year's autumnal equinox falls on a full moon. "

Spring equinox occurs around March 20 and Fall equinox occurs around September 22 for the Northern Hemisphere.

For the Southern Hemisphere these dates are opposite; the Spring equinox is around September 22 and the Fall equinox occurs around March 20.