phenomenon

A phenomenon is an extraordinary occurrence or circumstance. In the 1950s, rock-n-roll was considered a new cultural phenomenon, while today we think of crop circles as a mysterious phenomenon. An earthquake, for example, was a phenomenon, because you could see it (and hear it and feel it). Used in a sentence, “The class was excited to see the eclipse, as it was a natural phenomenon.”

"Phenomena" is simply the plural word for "phenomenon", or it is one or more extraordinary occurrences or circumstances. “May was a great month for astronomists because of the high amount of phenomena in the sky.”