Sandhi is a cover term for a wide variety of phonological processes that occur at morpheme boundaries. 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. It occurs particularly prominently in Sanskrit phonology, hence its naming with a word from that language, but most languages have it. Internal sandhi features the alteration of sounds within words at morpheme boundaries, as in sympathy (syn- + pathy). External sandhi refers to changes found at word boundaries, such as in the pronunciation for ten books, or the pronuciation of the as [ði] before a vowel (e.g. [ði æpl] for the apple). External sandhi effects can sometimes become morphologized (i.e. apply only in certain morphological and syntactic environments) and, over time, turn into consonant mutations.
Most tonal languages have tone sandhi, in which the tones of words alter in complicated ways. For example: Mandarin has four tones: a high monotone, a rising tone, a falling-rising tone, and a falling tone. In the common greeting ní hǎo, both words would normally have the falling rising tone. However, this is difficult to say, so the tone on nǐ mutates into ní.
