Just as there is formal and informal dress, so there is formal and informal speech. One is not supposed to turn up at a ministerial reception or at a scientific symposium wearing a pair of brightly coloured pyjamas. (Jeans are scarcely suitable for such occasions either, though this may be a matter of opinion.) Consequently, the social context in which the communication is taking place determines both the mode of dress and the modes of speech. When placed in different situations, people instinctively choose different kinds of words and structures to express their thoughts. The suitability or unsuitability of a word for each particular situation depends on its stylistic characteristics or, in other words, on the functional style it represents.
The term functional style is generally accepted in modern linguistics. Professor I. V. Arnold defines it as "a system of expressive means peculiar to a specific sphere of communication".
By the sphere of communication we mean the circumstances attending the process of speech in each particular case: professional communication, a lecture, an informal talk, a formal letter, an intimate letter, a speech in court, etc.
All these circumstances or situations can be roughly classified into two types: formal (a lecture, a speech in court, an official letter, professional communication) and informal (an informal talk, an intimate letter).
Accordingly, functional styles are classified into two groups, with further subdivisions depending on different situations.
Informal vocabulary is used in one's immediate circle: family, relatives or friends. One uses informal words when at home or when feeling at home.
Informal style is relaxed, free-and-easy, familiar and unpretentious. But it should be pointed out that the informal talk of well-educated people considerably differs from that of the illiterate or the semi-educated; the choice of words with adults is different from the vocabulary of teenagers; people living in the provinces use certain regional words and expressions. Consequently, the choice of words is determined in each particular case not only by an informal (or formal) situation, but also by the speaker's educational and cultural background, age group, and his occupational and regional characteristics.
Informal words and word-groups are traditionally divided into three types: colloquial, slang and dialect words and word-groups.
2. THE SUBDIVISION OF INFORMAL WORDS AND WORD-GROUPS
2.1 COLLOQUIAL WORDS
The term colloquial is old enough: Dr Johnson, the great English lexicographer, used it. Yet with him it had a definitely derogatory ring. S. Johnson thought colloquial words inconsistent with good usage and, considering as his duty to reform the English language, he advised “to clear it from colloquial barbarisms”. By the end of the 19th century with Neo-grammarians the description of colloquial speech came into its own, and linguists began to study the vocabulary that people actually use under various circumstances and not what they may be justified in using.
As employed in our time, the adjective colloquial does not necessarily mean ‘slangy’ or ‘vulgar’, although slang and vulgar vocabulary make part of colloquial vocabulary, or, in set-theoretical terminology, form subsets contained in the set we call colloquial vocabulary
Among other informal words, colloquialisms are the least exclusive: they are used by everybody, and their sphere of communication is comparatively wide, at least of literary colloquial words. These are informal words that are used in everyday conversational speech both by cultivated and uneducated people of all age groups. The sphere of communication of literary colloquial words also includes the printed page, which shows that the term "colloquial" is somewhat inaccurate.
Vast use of informal words is one of the prominent features of 20th century English and American literature.