Solar terms
The points in the solar year where a Saju month begins, set by the sun rather than the calendar.
In one sentence
The points in the solar year where a Saju month begins, set by the sun rather than the calendar.
How it is used
Twelve of them open the twelve month branches, and the start of spring opens the Saju year, which is why the year animal does not change at the calendar new year. A birth a few minutes either side of a term boundary gets a different month pillar, and the month pillar is the heaviest one in the element balance. It is also why a reading needs the hour and the place: the boundary is an exact moment, not a date.