As defined by Merriam-Webster's Dictionary, to plagiarize is:
"to steal and pass off (the ideas or words of another) as one's own : use (another's production) without crediting the source : to commit literary theft : present as new and original an idea or product derived from an existing source." Source: https://www.merriam-webster.com/dictionary/plagiarize
All of the following are considered plagiarism:
What DON'T you need to cite?
Most cases of plagiarism can be avoided, however, by citing sources. Simply acknowledging that certain material has been borrowed, and providing your audience with the information necessary to find that source, is usually enough to prevent plagiarism.