Medium originates from the Latin meaning middle, midst or means and this sense has carried through its various meanings.
The 16th century meaning of intervening substance gave rise to the 19th century meaning of enveloping substance, a term often used in scientific experiments.
Since the 18th century, medium has also meant intermediate agency (which no doubt is responsible for the medium who communicates with the other side).
The mass media also share this origin. Radio is a medium (or means) of communication. With the advent of national broadcasting of radio programs in the 1920s, the term mass media was born.
Given its Latinate origin, many people wonder about the correct plural of medium. Merriam Webster Online states that media is the correct plural of medium in almost all cases. However, many people also use media as a singular noun when referring to the mass media.
Here are some quotations from newspapers:
… platform, Periscope, as a gathering place for gamers. Facebook has also been scrambling to get in on the growing medium, courting individual gamers and adding the ability to tip streamers on Facebook Live as part of its “gaming … (www.nytimes.com)
News media finally calls out Trump on his lies. … Outrage over the Trump administration’s policy of separating children from their parents at the border is clearly growing, and it looks as if the media are a driving force behind it. … Finally, we’re saying what everyone has known for …
We’re quick to condemn the culture of entitlement for sports prima donnas, but we’ve created the same cozy cocoon for media ‘stars.’