In astrology, a cusp (which is the Latin for spear or point) is the imaginary line that separates a pair of successive signs in the zodiac or houses in the horoscope.
Since the solar disc has a diameter of approximately half a degree, it is possible for the Sun to bestride the cusp as it moves across the sky. When this occurs at the moment of birth such a person is said to be "born on the cusp" and some believe that their life is influenced by the characteristics of both signs.
Despite the fact that the term "cusp" is universally used for the boundaries of signs, not all astrologers agree that an object can ever be included in more than one sign. Many consider relevant only the location of the Sun's centre, which must be entirely in one sign, and would describe the natal Sun.
On the other hand, astrologers who consider objects "on the cusp" to be significantly different from objects entirely in one sign may relate such a description even when no part of the object crosses the boundary. That point of view may consider the Sun to be "on the cusp" even when its centre is as much as two degrees away from the sign boundary. They may also call other objects (much less than half a degree in diameter) "on the cusp" despite no part of the object being in the adjoining sign. Their claim is that the influence of the cusp gets weaker but does not all of a sudden disappear as the object gets further from the cusp.
A parallel debate applies to cusps between houses.
These are the following combinations of the Cusps:
Aries-Taurus Cusp - April 19 to April 24
Taurus-Gemini Cusp - May 19 to May 24
Gemini-Cancer Cusp - June 19 to June 23
Cancer-Leo Cusp - July 19 to July 24
Leo-Virgo Cusp - August 19 to August 24
Virgo-Libra Cusp - September 19 to September 24
Libra-Scorpio Cusp - October 19 to October 24
Scorpio-Sagittarius Cusp - November 19 to November 24
Sagittarius-Capricorn Cusp - December 19 to December 24
Capricorn-Aquarius Cusp - January 15 to January 22
Aquarius-Pisces Cusp - February 18 to February 22
Pisces-Aries Cusp - March 19 to March 26