The American Psychological Association has released a seventh edition of their publication manual. APA has several pages and handouts on their website that you may find useful that are linked below. To view more, click on the Style and Grammar Guideline link on the APA Style webpage.
Plagiarism is defined in the Texas A&M University-San Antonio's Student Handbook (November 2017) as:
"Plagiarism - The act of passing off some other person’s ideas, words, or work as one’s own. Examples include, but are not limited to, (1) The use, by paraphrase or direct quotation, of the published or unpublished work of another person without acknowledgement, documentation or citation; (2) The unacknowledged use of materials prepared by another person or agency engaged in the selling of term papers or other academic materials."
These websites below offer ways to better understand plagiarism and how to avoid it: