Equinoxes

Definition of Equinoxes

The two periods of the year (vernal equinox about March 21st, and autumnal equinox about September 22d) when the time from the rising of the sun to its setting is equal to the time from its setting to its rising.

That's the definition of Equinoxes in Black's Law Dictionary 6th Edition. Courtesy of Cekhukum.com.