Sandhi Dosha is a term used in Hindu astrology. It is based on the position of the planet (Rashi) in the 30 degree span.
The zodiac is divided into 12 signs. All the nine planets 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 Sandhidosha 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 into 27 nakshatras. Therefore, a planet also suffers from nakshatra sandhi dosha, when it is passing from one asterism to another.
Some of the other Sandhi dosha are Bhava Sandhi and Tithi Sandhi.
Bhava Sandhi happens when a planet passes from one house to another in horoscope.
Tithi Sandhi happens when the moon crosses a lunar day.