The description of the word you requested from the astronomical dictionary is given below.
aequinoctium = [Latin] equinox, from aequatio = [Latin] make equal, and noctium, nox = [Latin] night; plural equinoxes
The equinox is
the place in the sky between the stars where the Sun is during the vernal equinox. This location is also called the vernal equinox. In the equatorial and ecliptic coordinate systems, the vernal equinox has longitude and latitude equal to zero.
Because of the precession of the equinoxes, the equinox slowly moves between the stars, so when one quotes ecliptic or equatorial coordinates, one has to indicate relative to which equinox these coordinates are measured. Three equinoxes that are commonly used in stellar atlases and planetary calculations are those of 1950.0 (the beginning of the year 1950), 2000.0 (the beginning of the year 2000), and the equinox of the date (i.e., the equinox of the same date as the coordinates themselves).