The instants of the equinoxes are currently defined to occur when the ecliptic longitude of the Sun is either 0° or 180°. As the true motion of the Earth is affected by the gravitational pull of the Sun and Moon (and to lesser extent the other planets), there are tiny (up to 1¼ arcsecond) variations of the Sun's ecliptic latitude (discussed in section below) that may mean the Sun's center is not precisely over the equator at the moment of equinox.
On the day of an equinox, daytime and nighttime are of approximately equal duration all over the planet. They are not exactly equal, however, due to the angular size of the Sun and atmospheric refraction. The word is derived from the Latin aequinoctium, from aequus (equal) and nox (genitive noctis) (night).
The equinoxes are the only times when the solar terminator (the "edge" between night and day) is perpendicular to the equator. As a result, the northern and southern hemispheres are equally illuminated. The word comes from Latin aequus, meaning "equal", and nox, meaning "night".
In other words, the equinoxes are the only times when the subsolar point is on the equator, meaning that the Sun is exactly overhead at a point on the equatorial line. The subsolar point crosses the equator moving northward at the March equinox and southward at the September equinox.
The equinoxes, along with solstices, are directly related to the seasons of the year. In the northern hemisphere, the vernal equinox (March) conventionally marks the beginning of spring in most cultures and is considered the start of the New Year in Hindu calendar and the Persian calendar or Iranian calendars as Nowruz (means new day), while the autumnal equinox (September) marks the beginning of autumn.