Formal:
* Theft (emphasizes the illegal nature)
* Misappropriation (highlights the taking of something that belongs to another)
* Unauthorized use (focuses on the lack of permission)
* Infringement (implies violation of copyright)
* Duplication (simply means copying)
Informal:
* Copying (the most common and straightforward term)
* Borrowing (implies taking without permission)
* Stealing (more informal and direct)
* Passing off (means presenting someone else's work as your own)
More specific:
* Patchwriting (involves copying phrases and sentences and slightly altering them)
* Collage plagiarism (combines different sources without proper attribution)
* Self-plagiarism (using your own previous work without proper citation)
The best word to use will depend on the specific situation and the desired level of formality.