Etymology ( ET-im-OL-ə-jee) is the study of the origin and evolution of a word's semantic meaning across time, including its constituent morphemes and phonemes. It is a subfield of historical linguistics, and draws upon comparative semantics, morphology, semiotics, and phonetics.

For languages with a long written history, etymologists make use of texts, and texts about the language, to gather knowledge about how words were used during earlier periods, how they developed in meaning and form, or when and how they entered the language. Etymologists also apply the methods of comparative linguistics to reconstruct information about forms that are too old for any direct information to be available. By analyzing related languages with a technique known as the comparative method, linguists can make inferences about their shared parent language and its vocabulary. In this way, word roots in many European languages, for example, can be traced all the way back to the origin of the Indo-European language family.

Even though etymological research originated from the philological tradition, much current etymological research is done on language families where little or no early documentation is available, such as Uralic and Austronesian.