Etymology (/ˌɛtɪˈmɒlədʒi/ ET-im-OL-ə-jee) is the study of the origin and evolution of words—including their constituent units of sound and meaning—across time. In the 21st century a subfield within linguistics, etymology has become a more rigorously scientific study. Most directly tied to historical linguistics, philology, and semiotics, it additionally draws upon comparative semantics, morphology, pragmatics, and phonetics in order to attempt a comprehensive and chronological catalogue of all meanings and changes that a word (and its related parts) carries throughout its history. The origin of any particular word is also known as its etymology.

For languages with a long written history, etymologists make use of texts, particularly texts about the language itself, 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 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.

The word etymology is derived from the Ancient Greek word ἐτυμολογία (etymologíā), itself from ἔτυμον (étymon), meaning 'true sense or sense of a truth', and the suffix -logia, denoting 'the study or logic of'.

The etymon refers to the predicate (i.e. stem or root) from which a later word or morpheme derives. For example, the Latin word candidus, which means 'white', is the etymon of English candid. Relationships are often less transparent, however. English place names such as Winchester, Gloucester, Tadcaster share different forms of a suffix that originated as the Latin castrum 'fort'.

Reflex is the name given to a descendant word in a daughter language, descended from an earlier language. For example, Modern English heat is the reflex of the Old English hǣtu. Rarely, this word is used in reverse, and the reflex is actually the root word rather than the descendant word. However, this usage is usually filled by the term etymon instead. A reflex will sometimes be described simply as a descendant, derivative or derived from an etymon (but see below).^{[citation needed]}

Cognates or lexical cognates are sets of words that have been inherited in direct descent from an etymological ancestor in a common parent language. Doublets or etymological twins or twinlings (or possibly triplets, and so forth) are specifically cognates within the same language. Although they have the same etymological root, they tend to have different phonological forms, and to have entered the language through different routes.

A root is the source of related words within a single language (no language barrier is crossed). Similar to the distinction between etymon and root, a nuanced distinction can sometimes be made between a descendant and a derivative.