Sui generis
In local government, a "sui generis" entity is one that 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 (for historical or geographical reasons) very different from those of elsewhere in the country. 
In a press conference during which reporters were trying to analyse his political personality, Huey Long said, "...say that I am "sui generus", and let it go at that."
Sui generis
For example, in road traffic law, a statute may require consideration of large vehicles separately from other vehicles. The word "large" is ambiguous per se, but may be considered "heavy". Thus the relevant legislation (in Australian law) contains a section called "Terms used" or "Definitions" that itemises all words considered ambiguous, and confers specific interpretations consistent with natural language. So, this list tells us that "heavy vehicle" means a vehicle with a "GVM" over 4.5 tons;" and "GVM" means "gross vehicle mass"—the maximum loaded mass of the vehicle. Further explanations cover various contingencies. Thus "large" is equivalent to "heavy" and is (for this unique case) clearly defined, "sui generis".