In the taxonomical structure "genus → species" a species that heads its own genus is known as sui generis. This does not mean, however, that all genera with only a single member are composed of sui generis species. Only if the genus was specifically created to refer to that one species with no other known examples can the species be called sui generis. If the species is alone merely due to extinction, as in the case of the Homo genus, the surviving species is not sui generis, because other members of the genus are known, even if they are not currently extant.
In law, it is a term of art used to identify a legal classification that exists independently of other categorizations because of its singularity or due to the specific creation of an entitlement or obligation. Courts have used the term in describing cooperative apartment corporations, mostly because this form of housing is considered real property for some purposes and personal property for other purposes. When referring to case citations and authorities, lawyers (and Judges) may refer to an authority cited as being sui generis, meaning in that context, it is one confined (or special) to its own facts, and therefore may not be of broader application. This is also the modern view that courts are holding when deciding judgments based on Oil and Gas leases.
In the context of British law, the term means "unique".
In statutory interpretation, it refers to the problem of giving meaning to groups of words where one of the words is ambiguous or inherently unclear. For example, in criminal law, a statute might require a mens rea element of "unlawful and malicious" intent. Whereas the word "malicious" is well-understood, the word "unlawful" in this context is less clear. Hence, it must be given a meaning of the "same kind" as the word of established meaning.
This is particularly the case when the two or more words are conjoined, linked by the word "and", as opposed to placed in a disjunctive relationship, linked by the word "or". The interpretation of the two or more words might be different depending on the circumstances. Courts sometimes have to attribute a conjunctive (X and Y) intention to the legislature even though the list is disjunctive (X or Y) because, otherwise, no overall interpretation of the law in question would make sense.
In British town planning law, many common types of land use are classified in "Use Classes". Change of use of land within a Use Class does not require planning permission; however, changing between certain Use Classes, requires planning permission. Examples of sui generis use (identified in the Use Classes Order 1987) include embassies, theatres, amusement arcades, laundrettes, taxi or vehicle hire businesses, petrol filling stations, scrapyards, nightclubs, motor car showrooms, retail warehouses, clubs and hostels. It is a common misconception, even amongst qualified and experienced town planners, that the change of use from an existing use to one which is "Sui Generis" always requires planning permission. This is not correct. Although this is often the case, permission is only required if the sui generis use is materially different from the existing one. This will be a matter of fact and degree and professional opinion.
The term has been used in the context of Canadian Aboriginal law to describe the nature of Aboriginal title. Sui generis is also used in Aboriginal education to describe the work of Aboriginal people to define and create contemporary Aboriginal education as a "thing of its own kind". (Hampton, E. (p. 10-11) in Battiste & Barman (Eds.). First Nations Education in Canada: The Circle Unfolds. UBC Press, 1995) The motto "Sui Generis" has been adopted by the Akitsiraq Law School both in honour of the defining characteristic of aboriginal title in Canadian Law, and in acknowledgment of the unique form, admissions and curriculum of this one-of-a-kind professional legal education.
Generally speaking, protection for intellectual property is extended to matter depending upon its characteristics. The main types of intellectual property law -- copyrights, patents, and trademarks -- define characteristics, and any matter that meets such criteria is extended protection. However, there exist statutes in many countries that extend IP-type protection to matter that does not meet traditional definitions of protected intellectual property, such as intellectual property rights in mask works, ship hull designs, fashion designs in France, databases, or plant varieties.
The United States, Japan, and many EU countries protect the topography of semiconductor chips and integrated circuits under sui generis laws, some of whose aspects are borrowed from patent or copyright law. The U.S. law known as the Semiconductor Chip Protection Act of 1984 is codified at 17 U.S.C. §§ 901-915.
In political science, the unparalleled development of the European Union as compared to other international organizations has led to its designation as a sui generis geopolitical entity. There has been widespread debate over the legal nature of the EU given its mixture of intergovernmental and supranational elements, with the organisation thus possessing some characteristics common to confederal and federal entities.
A similar case which has led to the use of the label sui generis is the unique relationship between France and New Caledonia, since the legal status of New Caledonia can aptly be said to lie "somewhere between an overseas collectivity and a sovereign nation". Whereas there are perhaps other examples of such a status for other disputed or dependent territories, this arrangement is certainly unique within the French Republic.
In local government, a sui generis entity is one which does not fit with the general scheme of local governance of a country. For example in England, the City of London and the Isles of Scilly are the two sui generis localities, as their forms of local government are both very different from those of elsewhere in the country (for historical and geographical reasons).
The legal status of the Holy See has been described as a sui generis entity possessing an international personality.
In the sociology of Émile Durkheim, sui generis is used to illustrate his theories on social existence. Durkheim states that the main object of sociology is to study social facts. These social facts can only be explained by other social facts. They have a meaning of their own and cannot be reduced to psychological or biological factors. Social facts have a meaning of their own: they are 'sui generis'. Durkheim states that when one takes an organization and replaces some individuals with some others, the essence of the organization does not (necessarily) change. It can happen, for example, that over the course of a few decades, the entire staff of an organization is replaced, while the organization retains its distinctive character. Durkheim does not limit this thought to organization, but extends it to the whole society: he maintains that society, as it was there before any living individual was born, is independent of all individuals. His sui generis (its closest English meaning in this sense being 'independent') society will furthermore continue its existence after the individual ceases to interact with it.