While the word "plagiarism" itself is used to describe the act of stealing someone else's work, the suffix "-ism" generally denotes a belief, doctrine, or system.
Here are some examples:
* Capitalism: An economic system based on private ownership of the means of production.
* Socialism: A political and economic system based on collective ownership of the means of production.
* Communism: A political and economic system based on common ownership of the means of production.
Plagiarism is not a belief system or a doctrine, but rather a specific act.