A loanword is a word at least partly assimilated from one language (the donor language) into another language (the recipient or target language), through the process of borrowing. Borrowing is a metaphorical term that is well established in the linguistic field despite its acknowledged descriptive flaws: nothing is taken away from the donor language and there is no expectation of returning anything (i.e., the loanword).

Loanwords may be contrasted with calques, in which a word is borrowed into the recipient language by being directly translated from the donor language rather than being adopted in (an approximation of) its original form. They must also be distinguished from cognates, which are words in two or more related languages that are similar because they share an etymological origin in the ancestral language, rather than because one borrowed the word from the other.

A loanword is distinguished from a calque (or loan translation), which is a word or phrase whose meaning or idiom is adopted from another language by word-for-word translation into existing words or word-forming roots of the recipient language. Loanwords, in contrast, are not translated.

Examples of loanwords in the English language include café (from French café, which means "coffee"), bazaar (from Persian bāzār, which means "market"), and kindergarten (from German Kindergarten, which literally means "children's garden"). The word calque is a loanword, while the word loanword is a calque: calque comes from the French noun calque ("tracing; imitation; close copy"); while the word loanword and the phrase loan translation are translated from German nouns Lehnwort and Lehnübersetzung (German: [ˈleːnʔybɐˌzɛt͡sʊŋ] ^ⓘ).

Loans of multi-word phrases, such as the English use of the French term déjà vu, are known as adoptions, adaptations, or lexical borrowings.

Although colloquial and informal register loanwords are typically spread by word-of-mouth, technical or academic loanwords tend to be first used in written language, often for scholarly, scientific, or literary purposes.

The terms substrate and superstrate are often used when two languages interact. However, the meaning of these terms is reasonably well-defined only in second language acquisition or language replacement events, when the native speakers of a certain source language (the substrate) are somehow compelled to abandon it for another target language (the superstrate).^[relevant?]

A Wanderwort is a word that has been borrowed across a wide range of languages remote from its original source; an example is the word tea, which originated in Hokkien but has been borrowed into languages all over the world. For a sufficiently old Wanderwort, it may become difficult or impossible to determine in what language it actually originated.