In Writing Across Media, a medium is the style of making an argument. Examples of mediums are visual, audio, video, etc. Different mediums have different impacts on an argument. Not all arguments can be made through the same medium, either. For instance, making an audio argument about a brawl between two people would be very difficult. Certain things, like a fight between two people, are meant to be represented in video to be able to actually see what is happening. Having a video of a telephone call wouldn't make much sense because there's no visual component to go with it. Visual Evidence gives good examples of how visual evidence must be available to make a visual argument, otherwise you should use a different medium.