In astronomy, the winter solstice is the moment when the earth is in a point of its orbit at which the northern or southern hemisphere is most inclined away from the sun. This causes the sun to appear at its farthest below the celestial equator when viewed from earth. Solstice is a Latin borrowing and means "sun stand", referring to the appearance that the sun's noontime elevation change stops its progress, either northerly or southerly.

The date of the winter solstice is the date with the shortest day and the longest night of the year. In the northern hemisphere, this date usually falls on December 21/December 22, while it falls on June 21/June 22 in the southern hemisphere. These dates are also the dates of the summer solstice in the opposing hemisphere. At the summer solstice, the hemisphere is inclined towards the sun, and it appears to be at its farthest above the celestial equator.