The term cusp is used in astrology in order to refer to the border between either the signs of the zodiac or the twelve houses.
Originally a technical term that was used primarily by practitioners of astrology, in recent times it has come to be used more widely by the public as a result of its application in Sun-sign astrology.
Cusp is usually used in one of two ways:
1. To refer to the area around the border between the twelve signs of the zodiac. Technically the cusp itself is the dividing line between two signs, although when it is used within the context of Sun-sign astrology it is usually considered to be an area around the border, rather than as just the border itself. Within this context, when a person is said to be “born on the cusp” of a sign, it usually means that the Sun was either at the very beginning or the very end of a sign of the zodiac when they were born.
2. To refer to the border between each of the the twelve houses, especially in reference to the beginning of one house. This is the context in which the term cusp is used most frequently by astrologers. Within this context the cusp usually means the “beginning” of a specific house. This usage is important because it is the basis for determining the planetary rulers of the houses in a given chart. When the cusp or beginning of a certain house falls in a specific sign of the zodiac, the planet that rules that sign of the zodiac is said to become the ruler of the house as well.