Medium has two plurals—media (the Latin plural) and mediums. While there is some gray area between the plurals, they are kept separate in several contexts. Media is used in reference to mass communications, where media are newspapers, radio, the internet, and so on. It’s also used in science, where medium usually means an intervening substance through which something is transmitted. Mediums is the plural when medium refers to a person who communicates with the dead. In art, where medium refers to materials used to create a piece, both plurals are commonly used.
When media refers to mass communications in general, or when it’s used synecdochically to refer to groups of journalists or TV news crews, it is often treated as a mass noun, taking a singular verb. For example, one might say “the media is outside” instead of “the media are outside.”
Some people insist that Latin plurals must be used properly even in English. If you wish to try to please these people—which is up to each of us to decide for ourselves—use media as the plural in all contexts unrelated to communicating with the dead. But don’t listen to those who say mediums is wrong. Medium comes from Latin, but it is now a long-established English word (with roughly 500 years in the language), and we can treat it as one. The plural media stays in English only by convention, not because it’s more correct.
Media is preferred in science and in reference to mass communications, and it sometimes appears in art—for example:
‘Laser-like media’ is a technical term referring to those media that have inverted atomic populations. [Scientific American]
The media are equally diverse, including clay, fabric, glass, porcelain, water, steel, concrete, river grass and some toy soldiers. [Washington Post]
Broadcast media are especially inept—or uninterested—in reporting on science and technology. [The American]
Media is sometimes treated as a mass noun—for example:
He is the one the media is following from campaign stop to campaign stop. [Guardian]
“Not at all,” said Wyllie, explaining that the media was preoccupied with on-the-field matters. [Washington Post]
And mediums is usually used in reference to people who communicate with the dead, and it also shows up in other contexts—for example:
Both brought a new, unfettered approach to materials that pushed their respective mediums toward greater expressive freedom. [New York Times]
Maurice Barbanell polled numerous mediums (or more correctly, their spirit guides) to find out if there would be a war. [Independent]
The Festival of Lights programme is now available in two mediums—paperback and smartphone application. [Taranaki Daily News]