I received a résumé the other day in which the person listed that they have worked in a variety of “mediums”. That, to me, is like chalk on a blackboard.
The singular of media is medium. Newspaper, television, digital, radio and out-of- home, are, collectively, media. That is why the department was/is called the media department. Each of those means of communication is a medium. It pains me when I hear people in our industry talking about working in different mediums.
A few years ago I had an exchange of letters with one of the prominent art magazines when they wrote that a collage hanging in someone’s home was made of mixed mediums. The editor justified it as now common usage. However, About.com has a nice description of common usage. I Googled "medium vs media" and several web sites confirmed what I am writing. I did see one site which said that in advertising the two words were sometimes used interchangeably. Not not in my business or in my dictionary and clearly written by someone who never worked in a media department. In fact, I have never heard about the mediums department.
I would hate to see someone not get a job because they used the terms medias or mediums in their resume. While Latin is no longer taught (I took a semester in the eighth grade, but hated it and dropped out in favor of French, thank goodness), I think everyone should know the difference.