Two people who share mutual intelligibility of a language may have different style in using the language. It can be from the way how he speaks, the diction he chooses, or even the grammatical rules he uses. These cases become one of the concern areas of sociolinguistic study, namely language variation.
Talking about the language variation, it is divided into two kinds, internal and external. The first is simply defined as the variation of a language within the language itself, which means that, it refers to the variation in each level of constructions of language: phones, morphemes, words, phrases, clauses, sentences, paragraph, and finally meanings. Some English speakers will pronounce the word "today" as /tedaI/ and some pronounce it /tedeI/, this different shows the variation of language in the level of phone, phonological variation. Some also may say theirselves” for “themselves" to show a morphological variation in English. Or in many Southern dialects to find the word "done" used as an auxiliary, as in "she done already told you" or "I done finished a while ago.“ which become the cases of syntactic variation. Furthermore, semantic variation, variation in the level of meaning, has two kinds of cases in common, fist, same concept represented by two different terms, such as, football and soccer to refer to a kind of sport game played by two teams of eleven players using a round ball. Players kick the ball to each other and try to score goals by kicking the ball into a large net. Second, a term refers to two different concept, such as "Soda", British use it for seltzer water or club soda only, and American for any kinds of soft drink.
On the other hand, external variation refers to the variation of a language influenced by factors outside the language; it will lead us to the distinction between idiolect, dialect, sociolect, and chronolect.
All speakers of English can talk to each other and pretty much understand each other. Yet, no two of us speak exactly alike. Some differences are the result of age, sex, social situation, and where and when the language was learned. These differences are reflected in word choices, the pronunciation of words, and grammatical rules. The language of an individual speaker with its unique characteristics is referred to as the speaker’s idiolect. English may then be said to consist of anywhere from 450 million to 850 million idiolects, or the number of speakers of English (which seems to be growing every day and is difficult to estimate).
Like individuals, different groups of people who speak the same language speak it differently. Bostonians, New Yorkers, Texans, blacks in Chicago, whites in Denver, and Hispanics in Albuquerque all exhibit variation in the way they speak English. When there are systematic differences in the way groups speak a language, we say that each group speaks a dialect of that language. Dialects are mutually intelligible forms of a language that differ in systematic ways.
Suppose that there are two friends, Awin and Alice, who come from different regions, south and east of East Java Province, in which those two regions have the same language as their means of communication, reside in the same city located in the center of East Java Province, which also has the same language like the other two. However, when they communicate each other there are so many differences among them in using the language. Awin representing the east region people tends to use "rude" version of Javanese language which is strongly influenced by Surabaya style which is "very rude” and Alice tends to use "polite" version of Javanese since the people in her region is strongly influenced by Blitar style in their daily language. The different of geography shapes the variation of the language they use. This kind of variation which is influenced much by geographical factors is called dialect.
Among many languages in the world, Javanese is unique. it has three levels of politeness, Ngoko, Kromo Madya, and Kromo Inggil, each has its own vocabularies but similar in its rules and share mutual intelligibility. Moreover, in its society strata, the social of class is divided into three: priyayi, the term to call nobleman, santri is educated and religious people, and abangan is for both educated and uneducated but irreligious people. the highest strata uses kromo inggil and kromo madya in their daily communication but they also master ngoko, santri use kromo madya and ngoko and only few of them can speak kromo inggil; while abangan use ngoko as the dominant and kromo madya for only in certain occasions. When a Javanese talks to someone older or more honorable than him, he will use the highest level of politeness that he has mastered. He will use (1)'sare/ nendra' (kromo inggil) for sleep instead of (2)'tilem' (kromo madya) or (3)'turu'' (ngoko) when he speaks to his parent or boss, use the second to someone older than him, and third to someone of the same age. This variation of language which is influenced by social class in a society is called sociolect.
This term refers to variation of language due to the factor of time. by using diachronic method it can be discovered the variation of a language.