When you communicate, your purpose is not what you want to do; instead, it is what you want your audience to do as a result of reading what you wrote or listening to what you said. Thus ...

An audience is a person - or group of people ... an ideal reader - someone who will respond in a way that achieves the purpose of the writing. For example, someone who visits a place of interest ...

But purpose is not the only thing that affects this; the particular target audience is also important.