The main word for “unique” is “einzigartig” in German, literally meaning “one of a kind”. One can also use “einmalig”, but this word also means “one-time” or “one-off”:

– “Das ist ein einzigartiges Sammlerstück.” – “This is a unique collector’s item.”
– “Das ist eine einmalige Chance.” – “This is a unique opportunity.” (in the sense of a once-in-a-lifetime chance.)

As a noun, a “unique specimen” or a “unique copy” in German is “Unikat” (n.), (or, more rarely, “Unikum” (n.)):

– “Dieser Druck ist ein Unikat.” – “This is a unique print.”

Other words for “unique” are “ohnegleichen” (“unequalled” or “unprecedented”), “singulär” (“singular”) and in some senses “eindeutig” (“unambiguous”, for example in “uniquely defined”).