Cusp

The cusp is the starting point of an astrological house, that is, the boundary between two houses; in most house systems, the cusps of the 1st and 10th houses are the Ascendant and the Midheaven. In everyday usage, a person is also said to be “born on the cusp” when their birth falls in the last or first days of a sign. Astrologers point out that at the precise moment of birth, the Sun is always in a single sign, which only an exact calculation can determine.