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.
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
This tool from SUNY might be helpful: https://www.esc.edu/learning-support/thesis-generator/
