What's the adjective for singleton? Here's the word you're looking for.
Included below are past participle and present participle forms for the
verbs single, singularize and singularise which may be used as adjectives within certain contexts.
“This singular child of nature experienced all the vicissitudes of the most capricious fortune.”
“The author makes clear simply that, through writing, and without knowing it, he will say that which is singular to him.”
“He afterwards observes, that the universe is an effect completely singular and unparalleled, and consequently that no conclusion can be formed respecting the existence of a cause no less singular and unparalleled.”