Unicum, with its plural unica, is listed in a few dictionaries, including the Oxford Dictionary of English and Wiktionary.
It's quite a rare word: it seems to be mainly used by musicologists. For example, in The Organ Music of J S Bach, by Peter Williams: "It is difficult to see how any of this could have been applied again to another composition: the toccata must remain an unicum." And an example of the plural is in a publication called Anuario Musical, Volumes 19-20: "Certainly the facts that they are unica and the only known hymns by Gafforio set according to the Roman rather than the Ambrosian rite point strongly to this date." (Both examples found in Google Book Search.)
So, Gav, I'll add unicum and unica as allowable (rare) words the next time the word list is updated.
