Mutation or sandhi is the fusion of sounds across word boundaries, or 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).

The alternation of the English article "the" also exhibits mutation, because of the difficulty of saying /ðə æpl/ (the apple), we say /ðiː æpl/ instead.

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.

see also: Initial consonant mutation