Sandhi dosha, or Sandhi Dosham, is a term used in a horoscope in Hindu astrology. The zodiac is divided into 12 parts, called signs. All the nine planets, while moving along the ecliptic, span 30 degrees in each of the signs. While making predictions, strength of a planet is also obtained based on its longitude, varying from 0 to 30 degrees, in the sign occupied by it.
A planet which is near the middle of the sign is said to have maximum strength. It produces its maximum effect, good or bad. Otherwise, it produces weaker results in proportion to its distance from the middle. It suffers from rashi-sandhi-dosham when it has just entered into a sign or is about to leave it. Such a planet has almost nil strength and produces hardly any effects, good or bad, for the native.
The zodiac is also divided in 27 parts called nakshatras (asterisms). So, a planet suffers from the nakshatra-sandhi-dosham when it is passing from one asterism to another. In the case of the Moon, which suffers from both of these doshas, gandhamula is said to occur in the horoscope. This happens when it is passing from asterism Ashlesha to Magha (from Cancer to Leo), Jyeshta to Mula (from Scorpio to Sagittarius), or from Revati to Ashwini (from Pisces to Aries). These asterisms are called gandhamula-nakshatras, as they are not good for travel, marriage ceremony, etc. If the birth of a baby happens during gandhamula-nakshatras, inauspicious effects are believed to take place for different relations of the baby. Similarly, other types of sandhi dosham, bhava sandhi (when a planet passes from one house to another in horoscope) and tithi-sandhi (while the moon crosses a lunar day) occur. There are also believed to be inauspicious in nature.