The solstices are when the Sun is at its greatest distance from the celestial equator; the point in the apparent path of the Sun at which the Sun is farthest south of the equator. Solstice happens twice a year on about June 21 and December 21.
In other words, the solstice is when the tilt of the Earth's axis is in most inclined toward or away from the Sun, causing the Sun's apparent position in the sky to reach its northernmost or southernmost extreme. This makes the winter solstice the shortest day of the year, and summer summer solstice the longest.
solstices are when the it is the shortest day (December 22-25)
of the year or the longest (June 21-23).
Here is what the dictionary says. "1. Either of two times of the year when the sun is at its greatest distance from the celestial equator. The summer solstice in the Northern Hemisphere occurs about June 21, when the sun is in the zenith at the tropic of Cancer; the winter solstice occurs about December 21, when the sun is over the tropic of Capricorn. The summer solstice is the longest day of the year and the winter solstice is the shortest." The word solsitce is a noun.
It comes from a term that literally means "sun stand", because the sun reaches its highest or lowest point of the year on the solstice (highest on the summer solstice, lowest on the winter solstice).