A loanword is a lexical item taken over from one language into another with its form largely intact. In Haugen's (1950) classification it is the prototypical case of borrowing: morphemic importation without substitution. English café (from French), kindergarten (from German), tsunami (from Japanese), and yoga (from Sanskrit) are all loanwords. The word loanword is itself a calque of German Lehnwort.
A loanword rarely arrives unchanged. As it spreads through the recipient community it is adapted at several levels:
Degree of integration varies along a continuum from fully nativised forms whose foreign origin is no longer felt (English table, very, people from Old French) to recent loans still recognisably foreign (English schadenfreude, kimchi).
A loanword imports the form; a calque imports the structure but translates the parts. English skyscraper is a calque borrowed across dozens of languages (Dutch wolkenkrabber, Spanish rascacielos), where each language built a parallel compound from native roots. Had they instead taken skyscraper as a phonological import they would be loanwords. The distinction matters because loanwords carry visible foreign signatures while calques disappear into the native lexicon.
The Loanword Typology Project (Haspelmath and Tadmor 2009) compared borrowing patterns across 41 languages and roughly 1,500 meanings. Findings include a ranking of borrowability by semantic field, with religion, modern technology, and political organisation showing high borrowing rates and body parts, kinship terms, and basic verbs showing low rates. The project also confirmed Haugen's intuition that nouns are borrowed more readily than other word classes.
Loanwords give learners a free vocabulary head start when L1 has borrowed from L2. Vietnamese contains many English loanwords (marketing, online, check-in, taxi) and a substantial older French layer that overlaps with English Latinate vocabulary (bia from bière, cà phê from café). Useful classroom moves include surfacing these connections explicitly, contrasting Vietnamese pronunciation of borrowed items with English originals so learners can hear the gap, and watching for semantic drift where the loan no longer means in Vietnamese what it means in English.