1. Either of the times when the sun is farthest from the equator, on or about June 21 or December 21: The summer solstice falls between May and July in the northern hemisphere, and in the southern hemisphere in December, and vice versa for the winter solstice.

The summer solstice is the longest day of the year and the winter solstice is the shortest.
2. Either of the two points on the ecliptic when the sun reaches its northernmost or southernmost point relative to the celestial equator: When the solstice is at its position closest to the North Pole, the sun might shine the whole day if there aren't any clouds in the sky!