Plagiarism is defined by the University [pg. 1] as
the appropriation of another's idea, processes, results or words without giving appropriate credit.
The sanctions for this are severe. Make sure all your work can't be thought of as plagiarized by first referencing your work properly, the University has a tutorial here.
Second, if you really want to be sure, try submitting your essay or other work to turnitin.com. This is a service provided by the University for faculty and students to guard against plagiarism.
Finally, here is a .pdf of the entire policy.
[Dowlonad UL_Research_Misconduct_Policy.pdf]