An unclassified language is a language whose genetic affiliation to other languages has not been established. Languages can be unclassified for a variety of reasons, mostly due to a lack of reliable data but sometimes due to the confounding influence of language contact, if different layers of its vocabulary or morphology point in different directions and it is not clear which represents the ancestral form of the language. Some poorly known extinct languages, such as Gutian and Cacán, are simply unclassifiable, and it is unlikely the situation will ever change.
A supposedly unclassified language may turn out not to be a language at all, or even a distinct dialect, but merely a family, tribal or village name, or an alternative name for a people or language that is classified.
If a language's genetic relationship has not been established after significant documentation of the language and comparison with other languages and families, as in the case of Basque in Europe, it is considered a language isolate - that is, it is classified as a language family of its own. An 'unclassified' language therefore is one which may still turn out to belong to an established family once better data is available or more thorough comparative research is done. Extinct unclassified languages for which little evidence has been preserved are likely to remain in limbo indefinitely, unless lost documents or a surviving speaking population are discovered.
An example of a language that has caused multiple problems for classification is Mimi of Decorse in Chad. This language is only attested in a single word list collected ca. 1900. At first it was thought to be a Maban language, because of similarities to Maba, the first Maban language to be described. However, as other languages of the Maban family were described, it became clear that the similarities were solely with Maba itself, and the relationship was too distant for Mimi to be related specifically to Maba and not equally to the other Maban languages. The obvious similarities are therefore now thought to be due to borrowings from Maba, which is the socially dominant language in the area. When such loans are discounted, there is much less data to classify Mimi with, and what does remain is not particularly similar to any other language or language family. Mimi might therefore be a language isolate, or perhaps a member of some other family related to Maban in the proposed but as yet undemonstrated Nilo-Saharan phylum. It would be easier to address the problem with better data, but no-one has been able to find speakers of the language again.
It also happens that a language may be unclassified within an established family. That is, it may be obvious that it is, say, a Malayo-Polynesian language, but not clear in which branch of Malayo-Polynesian it belongs. When a family consists of many similar languages with great degree of confusing contact, a large number of languages may be effectively unclassified in this manner. Families where this is a substantial problem include Malayo-Polynesian, Bantu, Pama-Nyungan, and Arawakan.
There are hundreds of unclassified languages, most of them extinct, although there are some, albeit relatively few, that are still spoken; in the following list, the extinct languages are labeled with a dagger (+).
These languages are unclassifiable, not just unclassified, as we have nothing to classify. Sometimes a classification is assumed from ethnicity or location. Often, what is recorded of a supposed language amounts to little more than "other people used to live over there, and they spoke differently than we do". There are hundreds of such names in the literature, far too many to list here. (See, for example, a list of unclassified languages of South America.)
Many of these languages are also considered unclassifiable, as the amount of data may not be enough to reveal close relatives if there were some. For others there may be enough data to show the language belongs to a particular family, but not where within it, or to show the language has no close relatives, but not enough to conclude that it is a language isolate.
Some 'languages' turn out to be fabricated, as Kukurá of Brazil.