If something unusual happens, it can be described as a phenomenon. However, some people get confused with phenomena and phenomenon. That is because they do mean the same thing. Plus, they are both nouns. Sometimes, words that appear similar and have similar or exact definitions may be different because they are used differently in the sentence. That is to say, these words are both nouns.
The main difference between these words can be shown in an example. Many phenomena took place in the event. This indicates that there were more than one phenomena. Phenomena are plural, and the aspect is singular. Most of the time, the singular form of the word is used because there is usually only one unusual occurrence that is taking place.