The twenty-four solar terms divide the ecliptic into equal arcs of fifteen degrees, and this one falls at 60 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.
Xiaoman, the small fullness, refers to the grain beginning to fill but not yet full, and it pairs with Mangzhong that follows it as one of the sequences of paired terms running through the twenty-four. The pairing convention of a small and a great term recurs four times in the set, in the two heats, the two colds, the two snows and, differently constructed, here, and it gives the list much of its regularity.