Formal:
* Abstraction
* Broad statement
* Conclusion
* Deduction
* Inference
* Principle
* Rule
Informal:
* Assumption
* Oversimplification
* Sweeping statement
* Stereotype
Other:
* Induction
* Synthesis
The best synonym will depend on the specific meaning you want to convey. For example, "abstraction" might be a good choice if you are talking about a theoretical generalization, while "oversimplification" might be better if you are talking about a generalization that is too broad or inaccurate.