From BelieveTheSign

Plagiarism is the act of taking someone else's work or ideas and passing them off as one's own. It is all about not telling people where you got your ideas from, particularly if you copy someone word for word. It is not necessarily about copyright violation although that could be part of it.

The Bible records what the Lord thinks of prophets who plagiarize from each other, and say it is a word from the Lord:

Therefore, behold, I am against the prophets, saith the LORD, that steal my words every one from his neighbour. (Jeremiah 23:30 KJV)