In Astrology a “cusp” is the border between one sign or house and another. Thus lets say that someone has his Sun at 29 Pisces. Technically the person is Sun in Pisces, but really he or she is Sun Pisces cusp Aries. In other words he has a lot of Aries in him. The person is a not a pure Sun in Pisces. This is also true when looking at houses. A planet positioned late in the 2nd house (for example) within a degree or two of the 3rd house would not just be a factor in finances, but also in the intellectual life of the person.
If we observe nature we will understand cusps. Night and day are two discrete entities with very different properties. But let us observe the dawn or a sunset – a classic cusp situation. The dawn is breaking. We can see the Sun peep over the horizon. Is it day or is it still night? It is neither. It is this unique experience we call dawn. It partakes of both the qualities of day and night. A little bit of each. The same holds true with sunset. We watch the Sun gradually disappear beneath the horizon. Technically it is night fall. But is it really. There is still much light around. It doesn’t get really dark until a few minutes after the Sun disappears. So, again, we have this unique entity we call sunset. The cusp between night and day – but on the other end of dawn.
So a cusp situation is astrology is similar to the above. The planet is not completely in one area or the other. It is “in between”. It partakes of the nature of both houses or signs upon which it borders.