According to the Concise Oxford Companion to the English Language, the word "plagiarism" is from the Latin word plagiarius, which means "literary thief," and is defined as "The appropriation of someone's artistic, musical, or literary work for personal ends." Therefore, if something you write does not come from your own general knowledge, a citation is required to avoid plagiarism. You must give credit to the author for their opinions or ideas.
The most common forms of plagiarism include:
Photo Reference: Newbold, Curtis (2016, Aug 31). The Thirteen Types of Plagiarism in Order of Severity. The Visual Communication Guy Blog.