Plagiarism is thought by many to copy someone else's work or borrow someone else's unique ideas. However, terms such as "copy" and "borrowing" can pretend to be the importance of a crime.
According to the Merriam-Webster online dictionary, "plagiarism" means:
• Stealing (other people's thoughts and words) as your own
• Use (another work) without crediting the source
• Promise storybook theft
• Present ideas or products derived from existing sources as new originals
In other words, plagiarism is a fraud. This includes both stealing someone else's work and later lying about it.