The first letter of the alphabet is also a common English word that is virtually synonymous with one. As a word, a is the very antithesis of plurality.

This might help explain why there’s so much confusion about a group of words that I call “the -a team.” Here they are: bacteria, criteria, data, media, phenomena, Sierra. As you can see, all end in the letter a, which just sounds so darned singular that these words continue to confound even careful writers and speakers. Because the fact is, they’re all plural.

Bacteria Staphylococcus is a virulent form of bacteria. No problem there, but Staphylococcus is a virulent bacteria, well, now we have a problem. The singular is bacterium. So a sentence like The bacteria in the cut was infecting it is flawed—the bacteria were infecting it.

Criteria It’s the plural of criterion, a standard used for judging, deciding, or acting. The sentence Honesty is our chief criteria is ungrammatical; there can’t be only one criteria. Make it Honesty is our chief criterion or Honesty is one of our chief criteria. Your criteria are your standards, plural.

Those who know that criteria is plural aren’t out of the woods yet either: many believe the singular is “criterium.” And there are some who will reveal to you their “criterias.”

Data John B. Bremner, in Words on Words, states unequivocally, “The word is plural.” This one is thorny, because the singular, datum, is virtually nonexistent in English. Many people see data as a synonym for information, and to them, These data are very interesting sounds downright bizarre. Maybe, but it’s also correct. English scholar Theodore M. Bernstein says, “Some respected and learned writers have used data as a singular. But a great many more have not.”

Media Among the language’s most abused words is media, a plural noun; medium is the singular. A medium is a system of mass communication: The medium of television is a prominent component of the mass media.

Every day we hear and read statements like The media is irresponsible or The media has a hidden agenda. In those sentences, media should be followed by are and have.

There are some who prefer and defend the media is and the media has. To them, the various means of mass communication—newspapers, radio, TV, magazines, blogs, etc.—make up one “media.”

But writers should insist on the media are. It’s important that people think of the media as many voices, opinions, and perspectives rather than one monolithic entity.

Phenomena This troublemaker baffles even articulate speakers. Phenomena is plural; phenomenon is singular.

“Management is a universal phenomenon,” declares a business website. But a commentator on national television had it exactly backward. He spoke of “the phenomena of climate change” and later used phenomenon as a plural. Others say “phenomenas” when they mean phenomena.

Sierra Avoid “Sierras” when the topic is the vast California mountain range. An online camping guide says, “Translating from Spanish, sierra is plural in itself.” The Sierra Nevada Alliance, a conservation organization, elaborates: “The Sierra Nevada is a single, distinct unit, both geographically and topographically, and is well described by una sierra nevada. Strictly speaking, therefore, we should never pluralize the name—such as Sierras, or Sierra Nevadas, or even High Sierras …”

“Strictly speaking,” you say? What a concept!

—Tom Stern