What is the winter solstice?

In Japan, there is not only the four seasons, but also the concept of the "24 solar terms" which divide the year into 24 periods based on the movement of the sun. Among these are the "summer solstice", the longest day in summer, and the "winter solstice", the shortest day in winter. [What is the winter solstice?] The winter solstice is the day when the day is shortest and the night is longest in the year.