Sound change is a kind of language change concerning the phonological system (including phonetic realisation).
If a sound change is a general phenomenon, it is called a sound law or a phonetical rule. Exceptions to the law or rule can normally be explained by analogy. Some important sound laws were described by the Neogrammarians (e.g., Grimm's Law, Verner's Law for Germanic, Grassmann's Law for Sanskrit and Ancient Greek).
To describe sound changes, a special notation is used:
[-son][+voiced] > [-son][-voiced] /__#
= voiced non-sonorant sounds, i.e., voiced obstruents become voiceless at the end of a word (final devoicing).