From the Oxford English Dictionary:
A proposition laid down or stated, esp. as a theme to be discussed and proved, or to be maintained against attack; a statement, assertion, tenet.
From The American Heritage® Dictionary of the English Language, 4th Edition:
n. A proposition that is maintained by argument.
n. A dissertation advancing an original point of view as a result of research, especially as a requirement for an academic degree.
Your research question will be the main point of your writing. You should do some research before creating your research question. It should be clear so that your audience can easily identify what your purpose is. It should have focus so that you can answer the question thoroughly within the space of your writing assignment. Keep your research question as concise as possible. It should be complex. Develop an argument for your position by analyzing and synthesizing the evidence you found from your searching.
Perdue OWL - Creating a thesis statement and tips: https://owl.purdue.edu/owl/general_writing/the_writing_process/thesis_statement_tips.html
Guide to Writing Thesis Statements from the University of Washington: https://depts.washington.edu/pswrite/thesisstmt.html