"Idiolect" is a linguistic term referring to the specific patterns of language unique to an individual. Although large communities share languages, every individual varies, with a unique combination of grammatical patterns, vocabulary, pronunciation and content. The combination of these element's unique to a specific person is that person's idiolect. The idiolect is a linguistic concept rather than an observed phenomenon; different linguists use the term differently, and there is some debate about the concept.
The concept of the idiolect therefore identifies those patterns of speech which are specific to an individual person. The existence of this concept suggests that there is no universal language, merely the similar and overlapping idiolects of all the members of a linguistic community. Although the community's members speak in similar ways, each has a slightly different set of possible utterances from all the others. Some linguists therefore conceive of a language, not as a standard from which speakers deviate, but as the creation of overlapping individual idiolects.
The concept of the idiolect is primarily a theoretical part of linguistics, but it has some practical applications. Because each individual can be understood as having a unique combination of vocabulary and speech pattern, it is theoretically possible, given a large enough sample of speech or writing, to match a piece of speech or writing to the individual who created it. The field of forensic linguistics deals with this concept, attempting to demonstrate authorship linguistically as part of the process of criminal investigation.
Linguists do not always agree about the exact nature of an idiolect. Many linguists argue that although theoretically it is possible for an individual to have a unique set of possible utterances, language is a shared phenomenon. Many patterns of speech and vocabulary are not unique to an individual but are shared by a wider group, such as a nationality or subculture. Such a shared dialect is referred to as a sociolect.
The fact that so many linguistic variations are shared means, according to some linguists, that it is possible for two people to have identical sets of possible utterances. In such a case, the range of possible speech of the two people would be identical, meaning that neither had a unique pattern of language. Consequently, no idiolect would exist for either. In practice, although it is challenging to demonstrate that two people speak identically, it is simple to observe that many people have significant overlaps in their speech.