Plagiarism is "the action or practice of taking someone else's work, idea, etc., and passing it off as one's own" or also know as "literary theft" (OED Online).
It is a form of lying, cheating, theft, and fraud.
It is not a form of copying and borrowing -- that is a common misconception.
Plagiarism is a very serious offense.
"plagiarism, n." OED Online, Oxford University Press, March 2022, oed.com/view/Entry/144939. Accessed 9 March 2022.
Many things can be considered plagiarism. Here are a few examples (From Plagiarism.org):
“What Is Plagiarism?” Plagiarism.org, Turnitin LLC., 2017, plagiarism.org/article/what-is-plagiarism. Accessed 9 March 2022.
Most plagiarism cases are avoided if you cite your sources!
If you are ever unsure if you are plagiarizing, get a second opinion. Share what you wrote -- including the source you used -- with your instructor, a librarian, a writing center staff member, or other trusted individual.
When in doubt, always cite your source! Check out the Citation Style Guide for how to create and format a citation.