An artwork’s medium refers to the different materials or supplies that an artist utilizes in order to create a work of art. The plural of medium is media. Media can be a broader term, like "paint," or more specific, like "oil paint." All materials used can be considered part of the media. Artists use processes and techniques, as well as specialized tools with each different medium. For example, when discussing the medium of a painting, we can refer to both the type of paint used (oil, acrylic, watercolor, etc) and the base or ground to which the paint is applied (canvas, wood, paper, etc) as the media used. Knowing the medium when you look at a work of art is important to understanding and appreciating the art.
Art media have changed throughout history. Some materials were used in the past and are no longer used. Other materials have stayed the test of time. In the Renaissance, oil paint, watercolor and chalks were used. Also, a few engravings exist from this time (these are rare). Parchment was used as a substrate and is still used today, but is much less common. Art historians, authenticators, and appraisers all have detailed knowledge of the history of art media.
Dig into some broader categories of art media on the Tate Art Museum's website, but remember, the medium (or media) is what is is made out of, the process is how it is made.