A fellow asked if we had truly said a phenomena on an earlier program. We checked, of course, and (much to our relief) we had not. Phenomenon is the singular form of the plural that is (usually) phenomena. "Usually"? That's right. Phenomena is the preferred (and prevalent) plural when talking about more than one "fact or event that is observable, or that is susceptible to scientific description and explanation." Phenomena is also the plural used for more than one "object or aspect known through the senses rather than by thought or intuition," and for "rare or significant facts or events." (来源：英语学习门户 http://www.EnglishCN.com)

But when there is more than one "exceptional, unusual, or abnormal person, thing, or occurrence," the plural is sometimes (but not necessarily) phenomenons. We don’t know why this pattern exists, but it most definitely does, and it doesn't seem to excite much concern.

However, the use of phenomena where phenomenon might be expected does raise eyebrows. The singular phenomena has turned up in print occasionally over the past few centuries, and nowadays it is heard occasionally in speech. But although it follows a similar etymological pattern to the singular of the Latin-based nouns agenda, stamina, and candelabra, the singular of the Greek-based noun phenomena is still considered an unusual, questionable, or abnormal choice.