solstice

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Either of two times of the year when the sun is at its greatest angular distance from the celestial equator. The summer solstice in the Northern Hemisphere occurs about June 21, when the sun is at the zenith on the Tropic of Cancer; the winter solstice occurs about December 21, when the sun is at zenith on the Tropic of Capricorn. The summer solstice is the longest day of the year; the winter solstice is the shortest.

from The Century Dictionary.

• noun In astronomy:
• noun The time at which the sun is at its greatest distance from the equator, and when its diurnal motion in declination ceases, which happens about June 21st, when it enters Cancer (the summer solstice), and about December 22d, when it enters Capricorn (the winter solstice).
• noun A solstitial point. Hence Figuratively, culmination or turning-point; furthest limit.
• noun A stopping or standing still of the sun.

from the GNU version of the Collaborative International Dictionary of English.

• noun obsolete A stopping or standing still of the sun.
• noun The point in the ecliptic at which the sun is farthest from the equator, north or south, namely, the first point of the sign Cancer and the first point of the sign Capricorn, the former being the summer solstice, latter the winter solstice, in northern latitudes; -- so called because the sun then apparently stands still in its northward or southward motion.
• noun The time of the sun's passing the solstices, or solstitial points, namely, about June 21 and December 21. See Illust. in Appendix.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun One of the two points in the ecliptic at which the sun is furthest from the celestial equator. This corresponds to one of two days in the year when the day is either longest or shortest.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun either of the two times of the year when the sun is at its greatest distance from the celestial equator