The twenty-four solar terms divide the ecliptic into equal arcs of fifteen degrees, and this one falls at 150 degrees of the sun's apparent longitude. It is a major term, a zhongqi, one of the twelve that fall in the even positions of the twenty-four. The major terms are the ones that anchor the lunar months, and they alternate strictly with the sectional terms, so a zhong is always followed by a jie fifteen degrees later.
The terms have not always been spaced the same way. Until the seventeenth century they were reckoned by equal intervals of time, dividing the year into twenty-four equal parts, a method called pingqi. The Shixian calendar, prepared with Jesuit assistance and promulgated in 1645, redefined them by equal intervals of the sun's apparent longitude instead, fifteen degrees apart, which is the definition still used. Because the Earth's orbit is elliptical the sun does not move at a uniform apparent rate, so the intervals between terms now vary by about a day between the shortest and the longest.
Chushu, the ending of heat, is the major term of the seventh month in the ordinary numbering and marks the retreat of the summer. Its name is built on the same word for heat that gives Xiaoshu and Dashu, so three of the twenty-four terms share a root, which is a small illustration of how tightly the list is constructed as a set rather than assembled from unrelated observations.