Phenomenon means a fact of scientific interest that can be scientifically described, appraised, or explained: “The geography teacher explained the phenomenon of the earth’s movement around the sun with the help of aids.”
Phenomena is occasionally used as the singular form of phenomenon but such usage is considered incorrect: ” There are many phenomena in nature which one can’t understand with logic.”
Phenomenal means extraordinary: “The very first novel of the young writer proved a phenomenal success because the sale of its very first edition crossed a million.”