If you describe a person or thing as sui generis, you mean that there is no-one else or nothing else of the same kind and so you cannot make judgments about them based on other things.
[formal]
Japanese politics are sui generis.
sui generis in British English
(ˌsuːaɪ ˈdʒɛnərɪs)
adjective
unique
Word origin
Latin, literally: of its own kind
unique in British English
(juːˈniːk)
adjective
1.
being the only one of a particular type; single; sole
2.
without equal or like; unparalleled
3. informal
very remarkable or unusual
4. mathematics
a.
leading to only one result
the sum of two integers is unique
b.
having precisely one value
the unique positive square root of 4 is 2
▶ USAGE Unique is normally taken to describe an absolute state, i.e. one that cannot be qualified.Thus something is either unique or not unique; it cannot be rather unique or very unique. However, unique is sometimes used informally to mean very remarkable or unusual and this makes itpossible to use comparatives or intensifiers with it, although many people objectto this use