The term assimilation of a loan word is used to denote a partial or total conformation to the phonetic, graphical and morphological standards of the receiving language and its semantic system. The degree of assimilation depends upon the length of period during which the word has been used in the receiving language, upon its importance for communication purpose and its frequency. Oral borrowings due to personal contacts are assimilated more completely and more rapidly than literary borrowings, i.e. borrowings through written speech.
The following three groups may be suggested: completely assimilated loan words, partially assimilated loan words and unassimilated loan words or barbarisms. The group of partially assimilated words may be subdivided depending on the aspect that remains unaltered, i.e. according to whether the word retains features of spelling, pronunciation, morphology or denotation (when the word denotes some specific realia) that are not English.
I. Completely assimilated loan words are found in all the layers of older borrowings. They may belong to the first layer of Latin borrowings, e.g. cheese, street, wall or wine. Among Scandinavian loan words we find such frequent nouns as husband, fellow, gate, root, wing; such verbs as call, die, take, want and adjectives like happy, ill, low, odd and wrong. Completely assimilated French words are extremely numerous and frequent. Suffice it to mention such everyday words as table and chair, face and figure, finish and matter. A considerable number of Latin words borrowed during the revival of learning are at present almost indistinguishable from the rest of the vocabulary. Neither animal nor article differ noticeably from native words. The number of completely assimilated loan words is many times greater than the number of partially assimilated ones. They follow all morphological, phonetical and orthographic standards. Being very frequent and stylistically neutral, they may occur as dominant words in synonymic groups. They take an active part in word-formation. Moreover, their morphological structure and motivation remain transparent, so that they are morphologically analysable and therefore supply the English vocabulary not only with free forms but also with bound forms, as affixes are easily perceived and separated in series of loan words that contain them. Such are, for instance, the French suffixes -age, -ance and -ment, which provide speech material to produce hybrids like shortage, goddess, hindrance, speechify, and endearment.
II. The second group containing partially assimilated loan words can be subdivided into subgroups. The oppositions are equipollent (equal in force, power or validity).
(a) Loan words not assimilated semantically, because they denote objects and notions specific to the country from which they come. They may denote foreign clothing: mantilla, sombrero; foreign titles and professions: shah, rajah, sheik, bei, toreador; foreign vehicles: caique (Turkish), rickshaw (Chinese); food and drinks: pilaw (Persian), sherbet (Arabian); foreign currency: krone (Denmark), rupee (India), zloty (Poland), peseta (Spain), rouble (Russia), etc.
(b) Loan words not assimilated grammatically, for example, nouns borrowed from Latin or Greek which keep their original plural forms: bacillus : : bacilli; crisis : : crises; formula : : formulae; index : : indices; phenomenon : : phenomena. Some of these are also used in English plural forms, but in that case there may be a difference in stylistic terms, as in indices : : indexes.
(c) Loan words not completely assimilated phonetically. The French words borrowed after 1650 afford good examples. Some of them keep the accent on the final syllable: machine, cartoon, police. Others, alongside with specific stress, contain sounds or combinations of sounds that are not standard for the English language and do not occur in native words. The examples are: [Z] — bourgeois, camouflage, prestige, regime, sabotage; [wa:] — as in memoir, or the nasalized [R], [P] — melange. In many cases it is not the sounds but the whole pattern of the word phonetic transcription that is different from the rest of the vocabulary, as in some of the Italian and Spanish borrowings: confetti, incognito, macaroni, opera, sonata, soprano and tomato, potato, tobacco.
The pronunciation of words where the process of assimilation is phonetically incomplete will often vary, as in ['foiei] or ['fwaje] for foyer and ['bu:lva:], ['bu:lva:], ['bu:lava:], ['bu:lva:d] for boulevard. Eight different pronunciations are registered by D. Jones for the word fiance.
(d) Loan words not completely assimilated graphically. This group is fairly large and variegated. There are, for instance, words borrowed from French in which the final consonant is not pronounced, e. g. ballet, buffet, corps. Some may keep a diacritic mark: café, cliché. Specifically French digraphs (ch, qu, ou, etc.) may be retained in spelling: bouquet, brioche. Some have variant spellings.
It goes without saying that these sets are intersecting, i.e. one and the same loan word often shows incomplete assimilation in several respects simultaneously.
III. The third group of borrowings comprises the so-called barbarisms, i.e. words from other languages used by English people in conversation or in writing but not assimilated in any way, and for which there are corresponding English equivalents. The examples are the Italian addio, ciao 'good-bye', the French affiche for 'placard' and coup or coup d'Etat 'a sudden seizure of state power by a small group', the Latin ad libitum 'at pleasure' and the like.
For more detailed reading about borrowings see the following:
Loanwords. Major Periods of Borrowingin the History of English: