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

The day of the winter solstice is the shortest day and the longest night of the year.

In the northern hemisphere, the winter solstice usually falls on December 21/December 22, which is the southern hemisphere's summer solstice. At this time, the Sun appears over the Tropic of Capricorn, roughly 23.5 degrees South of the earth's equator. In the southern hemisphere, winter solstice falls on June 21/June 22, which is the northern hemisphere's summer solstice. At this time, the Sun appears over the Tropic of Cancer.