Cusp

A cusp is a boundary in a chart. There are two kinds: sign cusps, which are the fixed boundaries between adjacent zodiac signs (always at 0 degrees of each sign), and house cusps, which are the boundaries between adjacent houses and depend on the house system used and the moment of birth.

When someone says "I was born on the cusp of Leo and Virgo," they usually mean their Sun was placed near the boundary between those two signs. The Sun is in one sign or the other — there is no overlap and no in-between zone — but the person was born near the transition. This often produces a felt sense of belonging to both signs, especially because the Sun moves slowly enough that someone born within a few hours of the change may have other planets straddling the same boundary.

House cusps work differently. Because house systems vary, the cusps of houses 2 through 6 and 8 through 12 can sit at different ecliptic degrees depending on which system you use. A planet near a house cusp is one of the harder cases to interpret, because a small change in birth time or a switch in house system can move it from one house to the next.

The four "angular" house cusps — the first, fourth, seventh, and tenth — are the same in all house systems for any given birth: the ascendant, the IC, the descendant, and the midheaven. These four points are calculated from the geometry of the moment, not from the house system, which is why they remain stable across systems.