Etymology is the study of the history of words, their origins, and how their form and meaning have changed over time. By an extension, the term "etymology (of a word)" means the origin of a particular word.

For languages with a long written history, etymologists make use of texts in these languages and texts about the languages to gather knowledge about how words were used during earlier periods of their history and when they entered the languages in question. Etymologists also apply the methods of comparative linguistics to reconstruct information about languages 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 have been found that can be traced all the way back to the origin of, for instance, the Indo-European language family.

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