Advertising is more than just putting a poster on the wall. Advertising according to Wikipedia is a form of communication intended to persuade an audience (viewers, readers or listeners) to take some action.
I agree with Wikipedia to some extend on this one, I only have a slight adjustment.
To me, the definition is:
Advertising is a form of repetitive and consistent communication intended to inform and influence an audience (viewers, readers or listeners) to take some action.