noun

1. the moment or date when the sun in its apparent annual movement along the ecliptic reaches its maximum distance north or south of the celestial equator: in the Northern Hemisphere, the day of the summer solstice (about June 21, marking the beginning of summer) is the time of the sun's maximum elevation and, thus, has the longest period of sunlight; the day of the winter solstice (about Dec. 21, marking the beginning of winter) is the time of the sun's minimum elevation and, thus, has the shortest period of sunlight
2. either of the two points on the celestial sphere where the sun's path reaches its maximum distance north or south of the celestial equator

Etymology: ME < MFr < L solstitium < sol, the sun (see Sol) + sistere, caus. of stare, to stand