noun
1.
Astronomy .
a.
either of the two times a year when the sun is at its greatest distance from the celestial equator: about June 21, when the sun reaches its northernmost point on the celestial sphere, or about December 22, when it reaches its southernmost point.
b.
either of the two points in the ecliptic farthest from the equator.

2.
a furthest or culminating point; a turning point.