The law favors consistency, which is why new cases are decided based on comparison to prior similar cases. This is the doctrine of stare decisis. The notion of legal precedent dictates that similar cases should result in similar court decisions. Sometimes, however, a novel situation without precedent comes before our courts; such a matter would be regarded as sui generis, literally “of its own kind.” The term sui generis is used to describe a stand-alone thing or situation, something without precedent or comparison. The term can also be used to describe something that is peculiar or unique.
– BBC&B