“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.” (http://dictionary.com)
In other words, when it’s hanging around the North (summer) or South (winter) Pole.
Solstice; the Spring and Fall Equinox mark the midpoints between them. We count our seasons from those cardinal points along the Ecliptic, the Sun’s apparent path across the sky: 1• Aries (Spring); 1• Cancer (Summer); 1• Libra (Fall); and 1• Capricorn (Winter). We use the growing or receding light as metaphors for life itself; which is why the points at which we mark our journey are so significant in our thinking.
Solstices are the most visible of these points and hence dramatic. They are also cues: when the Sun is at its zenith, we are most active; when the Sun is only visible for a few hours of the day, we draw inward. Polar opposites, we say. And we are right.