Plagiarism is using the work of others and pretending it is your own. The Merriam Webster Dictionary defines plagiarizing as "to steal and pass off (the ideas or words of another) as one's own" (emphasis mine).
It is plagiarism if you copy someone else's words without giving them credit.
It is plagiarism if you make only light cosmetic changes to someone else's words, such as using a synonym.
Your professors want to see your work--not someone else's. This is your grade, so it needs to be your work.
At the very least you will not recieve credit for the work you turned in if it contained plagiarism. If your plagiarism is severe and/or you do it multiple times you can be expelled from the university.
Purdue OWL: Avoiding Plagiarism
Plagiarism from Indiana University Writing Tutorial Services
Citing Research and Avoiding Plagiarism from UTEP University Writing Center
Websites (programs and softwares) used by many Educational Institutions for plagiarism check:
List of some websites that offer free plagiarism detection:
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