Sandhi (Sanskrit: संधि sandhi "joining") is a cover term for a wide variety of phonological processes that occur at morpheme or word boundaries (thus belonging to what is called morphophonology). Examples include the fusion of sounds across word boundaries and the alteration of sounds due to neighboring sounds or due to the grammatical function of adjacent words. Sandhi occurs particularly prominently in the phonology of Indian languages (especially Telugu, Tamil, Kannada, Malayalam language, Marathi, Bengali and most importantly Sanskrit, which have complex sandhi rules), hence its name, but many other languages have it.
Read more about Sandhi: Types