the time at which the sun crosses the plane of the equator towards the relevant hemisphere, making day and night of equal length. It occurs about March 21 in the N hemisphere (Sept 23 in the S hemisphere)
2.
a. astronomy
the point, lying in the constellation Pisces, at which the sun's ecliptic intersects the celestial equator
b.
the time at which this occurs as the sun travels south to north (March 21)