Etymology is the study of the origins of words. Through old texts and comparison with other languages, etymologists reconstruct the history of words — when they entered a language, from what source, and how their form and meaning have changed.

In languages with a long written history, etymology makes use of philology, the study of old texts. However, etymologists also apply the methods of comparative linguistics to reconstruct information about languages that are too old for any direct information (such as writing) to be known. By analysing 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 which 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, nowadays much etymological research is done in language families where little or no early documentation is available, such as Uralic and Austronesian.

The word etymology itself comes from the Greek word for true meaning. The term was originally applied to the search of supposedly "original" or "true" meanings of words, on principles that are rejected as unscientific by modern linguistics. Pinder employed creative etymologies to flatter his patrons. Plutarch employed etymologies insecurely based on fancied resemblances in sounds. Isidore of Seville's Etymologiae was an encyclopaedic tracing of "first things" that remained uncritically in use in Europe until the fifteenth century. Etymologicum genuinum is a grammatical encyclopaedia edited at Constantinople in the ninth century, one of several similar Byzantine works. The fourteenth-century Legenda Aurea begins each vita of a saint with a fanciful excursus in the form of an etymology.