A solstice is an astronomical event that happens twice each year when the Sun's apparent position in the sky reaches its northernmost or southernmost extremes. The name 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 apparent movement of the Sun's path north or south comes to a stop before reversing direction.
http://en.wikipedia.org/wiki/Solstice