winter solstice

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

• noun In the Northern Hemisphere, the solstice that occurs on or about December 21.
• noun In the Southern Hemisphere, the solstice that occurs on or about June 21.

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

• noun astronomy The moment when the Earth is in that point of its orbit where the northern or southern hemisphere is most inclined away from the sun.

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

• noun December 22, when the sun is at its southernmost point