Loanwords are words adopted by the speakers of one language from a different language (the source language). A loanword can also be called a borrowing. The abstract noun borrowing refers to the process of speakers adopting words from a source language into their native language. "Loan" and "borrowing" are of course metaphors, because there is no literal lending process. There is no transfer from one language to another, and no "returning" words to the source language. The words simply come to be used by a speech community that speaks a different language from the one these words originated in.
Borrowing is a consequence of cultural contact between two language communities. Borrowing of words can go in both directions between the two languages in contact, but often there is an asymmetry, such that more words go from one side to the other. In this case the source language community has some advantage of power, prestige and/or wealth that makes the objects and ideas it brings desirable and useful to the borrowing language community. For example, the Germanic tribes in the first few centuries A.D. adopted numerous loanwords from Latin as they adopted new products via trade with the Romans. Few Germanic words, on the other hand, passed into Latin.
The actual process of borrowing is complex and involves many usage events (i.e. instances of use of the new word). Generally, some speakers of the borrowing language know the source language too, or at least enough of it to utilize the relevant word. They (often consciously) adopt the new word when speaking the borrowing language, because it most exactly fits the idea they are trying to express. If they are bilingual in the source language, which is often the case, they might pronounce the words the same or similar to the way they are pronounced in the source language. For example, English speakers adopted the word garage from French, at first with a pronunciation nearer to the French pronunciation than is now usually found. Presumably the very first speakers who used the word in English knew at least some French and heard the word used by French speakers, in a French-speaking context.
Those who first use the new word might use it at first only with speakers of the source language who know the word, but at some point they come to use the word with those to whom the word was not previously known. To these speakers the word may sound 'foreign'. At this stage, when most speakers do not know the word and if they hear it think it is from another language, the word can be called a foreign word. There are many foreign words and phrases used in English such as bon vivant (French), mutatis mutandis (Latin), and Fahrvergnuegen (German).
However, in time more speakers can become familiar with a new foreign word or expression. The community of users of this word can grow to the point where even people who know little or nothing of the source language understand, and even use, the novel word themselves. The new word becomes conventionalized: part of the conventional ways of speaking in the borrowing language. At this point we call it a borrowing or loanword.
(It should be noted that not all foreign words do become loanwords; if they fall out of use before they become widespread, they do not reach the loanword stage.)
Conventionalization is a gradual process in which a word progressively permeates a larger and larger speech community, becoming part of ever more people's linguistic repetoire. As part of its becoming more familiar to more people, a newly borrowed word gradually adopts sound and other characteristics of the borrowing language as speakers who do not know the source language accommodate it to their own linguistic systems. In time, people in the borrowing community do not perceive the word as a loanword at all. Generally, the longer a borrowed word has been in the language, and the more frequently it is used, the more it resembles the native words of the language.
English has gone through many periods in which large numbers of words from a particular language were borrowed. These periods coincide with times of major cultural contact between English speakers and those speaking other languages. The waves of borrowing during periods of especially strong cultural contacts are not sharply delimited, and can overlap. For example, the Norse influence on English began already in the 8th century A.D. and continued strongly well after the Norman Conquest brought a large influx of Norman French to the language.
It is part of the cultural history of English speakers that they have always adopted loanwords from the languages of whatever cultures they have come in contact with. There have been few periods when borrowing became unfashionable, and there has never been a national academy in Britain, the U.S., or other English-speaking countries to attempt to restrict new loanwords, as there has been in many continental European countries.
The following list is a small sampling of the loanwords that came into English in different periods and from different languages.
I. Germanic period
II. Old English Period (600-1100)
(few ordinary words, but thousands of place and river names: London, Carlisle,
III. Middle English Period (1100-1500)
Also Middle English French loans: a huge number of words in age, -ance/-ence, -ant/-ent, -ity, -ment, -tion, con-, de-, and pre- .
Sometimes it's hard to tell whether a given word came from French or whether it was taken straight from Latin. Words for which this difficulty occurs are those in which there were no special sound and/or spelling changes of the sort that distinguished French from Latin
IV. Early Modern English Period (1500-1650)
The effects of the renaissance begin to be seriously felt in England. We see the beginnings of a huge influx of Latin and Greek words, many of them learned words imported by scholars well versed in those languages. But many are borrowings from other languages, as words from European high culture begin to make their presence felt and the first words come in from the earliest period of colonial expansion.
Greek (many of these via Latin)
V. Modern English (1650-present)
Period of major colonial expansion, industrial/technological revolution, and American immigration.
Words from European languages
Yiddish (most are 20th century borrowings)
Words from other parts of the world
American Indian languages