Plagiarism is "...using or closely imitating the language and thoughts of another author without authorization and the representation of that author's work as one's own..."
Dictionary.com. Retrieved September 22, 2025, from Dictionary.com.
If you buy a paper from a web site, written by someone else and try to pass it off as your writing, that's obviously intentional plagiarism. Not all plagiarism may be intentional, however.
If you copy and paste all or part of a sentence from an online source and don't attribute it to the person who wrote it, that's plagiarism.
If you use the same basic sentences from someone else's writing and simply substitute a word here and there to make it sound different, that's plagiarism.