Define an equinox?

An equinox is a phenomenon that occurs twice a year when the sun crosses the celestial equator and day and night are of approximately equal length all over the world. This marks the beginning of spring and autumn.

An equinox is an astronomical event in which the Sun is directly above Earth's equator, causing the day and night to be of equal length. It occurs twice a year, once in March (vernal equinox) and once in September (autumnal equinox).