Function: noun
Inflected Form(s): plural phe·nom·e·nas
Date: 1576
nonstandard : phenomenon
usage Phenomena has been in occasional use as a singular for more than 400 years and its plural phenomenas for more than 350. Our evidence shows that it is primarily a speech form used by poets, critics, and professors, among others, but one that sometimes turns up in edited prose <the Borgia were, in modern terms, a media phenomena - Economist>. It is etymologically no more irregular than stamina, agenda, and candelabra, but it has nowhere near the frequency of use that they have, and while they are standard, phenomena is still rather borderline.
[M-W's Col. Dic.]

1: Does 'speech form' stand for word or expression in the given context?
2: What is an edited prose? A literature which has been checked carefully to conform to the correct rules and usage of a language?

Yes, and yes.

Thanks a lot for the prompt reply! It seems the forums is running slow these days - it takes quite a time to get the replies.