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.
Examples of PHENOMENON
natural phenomena like lightning and earthquakes
the greatest literary phenomenon of the decade
The movie eventually became a cultural phenomenon.
For example, we talk more loudly in cars, because of a phenomenon known as the Lombard effect—the speaker involuntarily raises his voice to compensate for background noise. —John Seabrook, New Yorker, 23 June 2008
This follow-the-winemaker phenomenon is a unique wrinkle in our wine culture. —James Laube, Wine Spectator, 15 May 2008
Contrary to the notion that war is a continuation of policy by other means … , both Keegan and Mueller find that war is a cultural product rather than a phenomenon or law of nature and therefore subject, like other modes of human expression (the wearing of togas or powdered wigs, the keeping of slaves, the art of cave painting), to the falling out of fashion. —Lewis H. Lapham, Harper's, September 2007
The days and nights of the Irish pub, smoky and dark and intimate, are giving way to another phenomenon: the superpub. These are immense places, loud with music; part honkytonk, part dance hall, some servicing as many as a thousand drinkers on several floors. —Pete Hamill, Gourmet, April 2007
They were ephemera and phenomena on the face of a contemporary scene. That is, there was really no place for them in the culture, in the economy, yet they were there, at that time, and everyone knew that they wouldn't last very long, which they didn't. —William Faulkner, letter, 7 Mar. 1957