The Century Dictionary and Cyclopedia defines [Singular]( as meaning:

> 7. Out of the usual course; unusual, uncomon; somewhat strange; a little extraordinary: As a singlular phenomena

More literally it derives from single, meaning one, so a singular phenomena is logistically something that is assessed as happening just once. It is of course possible to use the term hyperbolicly or mistakenly, which can diminish the effect, but I doubt you will find any word that lacks that particular problem.

Since the dictionary entry was written, [Singular Event actually overtook singular phenomena in popularity]( as shown by google Ngrams, and collins shows that [singular]( is one of the top 10,000 words used in the language, so I figure that this demonstrates that it is more than common and easily enough understood.