The plural of phenomenon is phenomena or phenomenons.
1. An interesting or unusual fact or event which is difficult to completely explain or interpet.
2. An exceptional person, object or occurance.
The word phenomenon is a singular noun and it has two forms of plural: phenomena and phenonemons.
The word phenomena is much more commonly used as the plural of phenomenom in both definitions of the word. The word phenomenons is used for the second definition of the word, when referring to exceptional people, objects or occurances.
This form of the word is rarely used, especially in written English as you can see from the graph below.
Singular Example: The northern lights are a naturally occuring phenomenon caused by solar wind. You can even see it from space!
Plural Example: Soccer players Messi and Ronaldo are two phenomenons of the most popular sport in the world. Which player of the two is the greatest is a hotly debated subject!
The graph shows the occurances of the plural of phenomenon in written English since 1800 using Google's Ngram Viewer.
Some of the world's lesser known phenomena include 'fire rainbows', 'fire tornados', and 'desert roses'.