Identifying your audience
Writing is communication
In academic writing, the purpose of the text is usually to convey new information to other people within the field. This information can take various forms: new data, suggestions for a new approach, a new interpretation, etc. Writers therefore need to take their intended readers' knowledge of the subject into consideration.
For reasons of integrity (such as the importance of avoiding bias), as well as for reasons of clarity, you need to know who you are writing for. This is something that is often discussed in essay courses, and if you are uncertain about how to tackle the question of potential readers, this is something you should raise with your supervisor.
Who do you write for?
In undergraduate writing (short essays, take-home exams, response papers, etc.), the intended reader is usually the teacher and peers of the student. In degree projects and theses, the audience is more diverse. A degree project in engineering or in law, written in cooperation with a company or a non-governmental organization, for instance, may have a more clearly defined readership than a degree essay within, say, history or comparative literature.
Identifying potential or actual readers of your text will affect the kind and amount of background information that you provide in your text. For example, anyone who reads a scientific paper published in a medical journal will be expected to have enough background knowledge of anatomy so that writers can refer to, say, a particular muscle or organ without having to state where it is located in the body. A reader of an essay or article in the field of literature or history will similarly be expected to be acquainted with terminology specific to those fields.