General synonyms:
* Theft
* Copying
* Imitation
* Borrowing (when used without proper attribution)
* Appropriation
* Duplication
* Counterfeiting
More specific synonyms:
* Literary theft
* Academic dishonesty
* Cheating
* Fraud (in academic or professional contexts)
* Passing off (presenting someone else's work as your own)
Informal synonyms:
* Ripping off
* Stealing
* Lifting (for specific passages or ideas)
The best synonym to use will depend on the specific context and the level of formality you require.