According to Wikipedia:
A solstice is an astronomical event that happens twice each year as the sun reaches its highest or lowest excursion relative to the celestial equator on the celestial sphere.
As a result, on the solstice the sun appears to have reached its highest or lowest annual altitude in the sky above the horizon at local solar noon. The word solstice is derived from the Latin sol (sun) and sistere (to stand still), because at the solstices, the sun stands still in declination; that is, the seasonal movement of the sun's path (as seen from Earth) comes to a stop before reversing direction. The solstices, together with the equinoxes, are connected with the seasons. In many cultures the solstices mark either the beginning or the midpoint of winter and summer.
The term solstice can also be used in a broader sense, as the date (day) when this occurs. The day of the solstice is either the longest day of the year (in summer) or the shortest day of the year (in winter) for any place outside of the tropics.Solstice is one of our favorite times of year—in the heart of the Sonoran Desert it's blazingly hot (106 today), incredibly dry (6% humidity), and often windy. But the black-throated sparrows are singing madly, the summer Anna's hummingbirds are arriving back from California, the nighthawks and elf owls are trilling and peeping all night long—and it means the rains are coming, the desert is ready, the anticipation is palpable.