Language change is variation over time in a language's features. It is studied in several subfields of linguistics: historical linguistics, sociolinguistics, and evolutionary linguistics. Traditional theories of historical linguistics identify three main types of change: systematic change in the pronunciation of phonemes, or sound change; borrowing, in which features of a language or dialect are altered as a result of influence from another language or dialect; and analogical change, in which the shape or grammatical behavior of a word is altered to more closely resemble that of another word.
All living languages are continually undergoing change. Some commentators use derogatory labels such as "corruption" to suggest that language change constitutes a degradation in the quality of a language, especially when the change originates from human error or is a prescriptively discouraged usage. Modern linguistics rejects this concept, since from a scientific point of view such innovations cannot be judged in terms of good or bad. John Lyons notes that "any standard of evaluation applied to language-change must be based upon a recognition of the various functions a language 'is called upon' to fulfil in the society which uses it".