“Phenomenon” comes to English from Greek through Latin. According to Etymonline, in Greek the word meant “that which is seen or appears,” so essentially the same thing it means today. The singular is ‘phenomenon.’ The plural is ‘phenomena.’ Its meaning hasn’t changed, and you still make it plural like you make Greek words plural.

Although phenomena is often treated as if it were singular, correct usage is to employ phenomenon with a singular construction and phenomena with a plural: that is an interesting phenomenon (not phenomena); several new phenomena were recorded in his notes

By far the most common and universally accepted plural form is the classical phenomena; the Anglicised phenomenons is also sometimes used. The plural form phenomena is frequently used in the singular, and the singular form is sometimes used in the plural.

Phenomena has been in occasional use as a singular since the early 18th century, as has the plural phenomenas.