phenomena

Phenomena has been in occasional use as a singular since the early 18th century, as has the plural phenomenas. Our evidence shows that singular phenomena is primarily a speech form used by poets, critics, and professors, among others, but one that sometimes turns up in edited prose. Although it seemed like a fad a few years ago, Twitter has evolved into a phenomena with more than 200 million users … — Myron P. Medcalf It is etymologically no more irregular than stamina and agenda, but it has nowhere near the frequency of use that they have, and while they are standard, phenomena is still rather borderline.