The Century Dictionary and Cyclopedia defines [Singular](http://www.micmap.org/dicfro/previous/century-dictionary/5648/7/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](https://books.google.com/ngrams/graph?content=singular+event%2Csingular+phenomena&year_start=1800&year_end=2008&corpus=15&smoothing=3&share=&direct_url=t1%3B%2Csingular%20event%3B%2Cc0%3B.t1%3B%2Csingular%20phenomena%3B%2Cc0#t1%3B%2Csingular%20event%3B%2Cc0%3B.t1%3B%2Csingular%20phenomena%3B%2Cc0) as shown by google Ngrams, and collins shows that [singular](https://www.collinsdictionary.com/dictionary/english/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.