In etymology, two or more words in the same language are called doublets or etymological twins or twinlings (or possibly triplets, and so forth) when they have different phonological forms but the same etymological root. Often, but not always, the words entered the language through different routes. Given that the kinship between words that have the same root and the same meaning is fairly obvious, the term is mostly used to characterize pairs of words that have diverged at least somewhat in meaning. For example, English pyre and fire are doublets with merely associated meanings despite both descending ultimately from the same Proto-Indo-European (PIE) word *péh₂ur.

Words with similar meanings but subtle differences contribute to the richness of modern English, and many of these are doublets. A good example consists of the doublets frail and fragile. (These are both ultimately from the Latin adjective fragilis, but frail evolved naturally through its slowly changing forms in Old French and Middle English, whereas fragile is a learned borrowing directly from Latin in the 15th century.)

Another example of nearly synonymous doublets is aperture and overture (the commonality behind the meanings is "opening"). But doublets may develop divergent meanings, such as the opposite words host and guest, which come from the same PIE word *gʰóstis and already existed as a doublet in Latin, and then Old French, before being borrowed into English. Doublets also vary with respect to how far their forms have diverged. For example, the connection between levy and levee is easy to guess, whereas the connection between sovereign and soprano is harder to guess.