phenomena

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.