sui generis
adjective·/ˌsuː.aɪ ˈdʒɛn.ər.ɪs/
Of its own kind, belonging to no established class, resisting comparison not by superiority but by singularity. Where unique can be casual praise, sui generis is taxonomic: a label for what refuses the label.
Her work was sui generis—recognizably itself before it was recognizably anything else.
Etymology
From Latin sui "of itself" + generis "of a kind, genus." A tidy paradox: classification used to declare the unclassifiable.
Related Words
incomparablesingular by relation to others
idiosyncraticdistinctive in personal ways
unprecedentedwithout prior example
hapaxonce-only in a corpus; rarity in evidence