The twenty-four solar terms divide the ecliptic into equal arcs of fifteen degrees, and this one falls at 225 degrees of the sun's apparent longitude. It is a sectional term, a jie, one of the twelve that fall in the odd positions of the twenty-four and open the twelve solar months. The sectional terms and the major terms alternate strictly around the year, so a jie is always followed by a zhong fifteen degrees later.
The twelve major terms are what make the Chinese calendar work as a lunisolar one. Each ordinary lunar month is expected to contain exactly one major term, and the month takes its number from the term it contains, so the eleventh month is by definition the one holding the winter solstice. When a lunar month falls entirely between two major terms and contains none, it is declared intercalary and repeats the number of the month before it, which is how a thirteenth month is inserted without any fixed rule about when.
Lidong opens the winter at 225 degrees, the last of the four season-opening terms. In several regions it carries its own food customs, and in northern China it is associated with the eating of dumplings, though the observances attached to the individual terms vary a great deal by place and the atlas does not present any single set as standard.