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The Century Dictionary and Cyclopedia defines Singular as meaning:

  1. 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.