The business of designing and writing advertisements for publication or broadcast. Dictionary guys cheat by using a root word, advertisements, in their definition. You all know what it is and what it's supposed to do. Advertising in and of itself is not a medium. But the media as we know them in the U.S. could not and would not exist without advertising.
Advertising is a fueler of all the media.
However, it's a two-way street. Without the media, there wouldn't be advertising. So, almost without exception, Macaulay's principle holds true for businesses today, and it is especially true for the mass media. Their solvency as businesses depends to a great extent on advertising, and advertising depends heavily on the mass media as its vehicle. It is impossible to imagine the American mass media without advertising, for they have grown up together and each depends on the other.