You will start with a broad definition of your topic. The topic will evolve and develop and, in time, take on a different shape. The definition of your topic / question is a flexible and ongoing process. What do you need to keep in mind? Above all, the need to revisit or even revise your topic's focus when necessary!
You might be ready now to settle on the topic of your choice. To define your topic means to identify content and questions and to start putting limits around your topic. The definition of your topic is essential to formulating a research strategy. This strategy will help you focus your bibliographical search (or search for other kinds of non-printed material) to find the relevant material. It will help you to plan and progress.
You are now at the (re)defining stage. It is important to take a flexible approach to the definition so that it can take shape as you go along -widening the process of knowledge while narrowing your focus to produce a manageable, worthwhile thesis.
What is the overall focus of your topic and research question(s)?
What is the scope you are considering?
NB You may be reasonably happy with the focus and scope of your research proposal and wish to take it forward - go for it! Talk to your supervisor(s) if you feel you may need to spend more time getting the balance right or if you have serious doubts about the scope and focus of your topic or question(s). Perhaps you may need to re-evaluate your question or even topic. Do talk to your supervisor(s) whatever conclusion you come to.