A simple google search of the term medium will bring a slew of definitions and example sentences mainly involving means of communication media. Now, when we think of the term medium, almost certainly we all think of a vehicle of communication or a means of conveying an idea. Oxford dictionary’s current definition of the term reads, “A means by which something is communicated or expressed". Similarly, Merriam Webster’s most current definition is also in terms of communication media, it reads “one of the means or channels of general communication, information, or entertainment in society, as newspapers, radio, or television”. If compared to Dictionary.com’s definition of the termmedia, which reads, “the means of communication, as radio and television, newspapers, and magazines, that reach or influence people widely”, one will find that the two definitions are so closely related that the terms appear synonymous. But before this term was associated so closely with means of communication, its meaning has varied over time. The term originated around 1575-1585 from the latin term medius, denoting something intermediate in nature or degree. A dramatic increase in the use of the term medium was seen in the late 1900s-early 2000s, when we as a culture began to use the term to refer to vehicles of communication. When and why this shift in meaning occurred is a meaningful subject of research which McLuhan touches on in his excerpt “The Medium is the Message”. McLuhan's main objective is too assert that the medium of communication chosen is as essential to understand as the message being communicated. McLuhan's broad use of the term medium as "any extension of ourselves", opens up the door to understanding and awareness that mediums of communication are vast and all around us.
References
http://www.etymonline.com/index.php?term=medium
http://www.oxforddictionaries.com/us/definition/american_english/medium
http://www.merriam-webster.com/dictionary/medium
--Julie