What are the "solar terms" and "neutral gas" of solar terms?

The 24 solar terms are beginning of spring, Rain, Surprise, Equinox, Qingming, Grain Rain, Long Summer, Xiaoman, Busy, Summer Solstice, Slight Summer, Great Summer, beginning of autumn, Chushu, Millennium, Equinox, Cold Dew, Early Frost, beginning of winter, Light Snow, Heavy Snow, Winter Solstice, Slight Cold and Severe Cold. In this order, all the single rooms are "solar terms", usually called "solar terms", while the double rooms are "neutral gas", referred to as "qi", collectively referred to as "solar terms". Now it is generally called twenty-four solar terms.