For a long time, the plural of “medium” meaning a means of communication was “media.” So you might ask, “What medium do you as a painter prefer?” And the answer might be, “My favorite media are tempera and watercolors.” And then TV followed on the footsteps of radio and newspapers, giving us three major media for public communication, and we called them “the mass media.”
Lazy creatures that we humans are, pretty soon it became “the media.” But we curmudgeonly types continued to complain that the word had more meanings than just newspapers, radio, and TV.
Well, it looks like we’re losing the battle. Here’s National Public Radio, in writing:
Pulitzer judges whittled their winning group from a vast number of possibilities, and the works they’ve chosen represent a vast array of styles, mediums, and much more.
I guess the plural of “medium” is going to be “mediums.”