General:
* Expansion: This is a good general term for broadening something.
* Enlargement: Similar to expansion, but emphasizing making something bigger.
* Widening: This implies extending something in a specific direction.
Specific:
* Scope: This refers to the breadth or extent of something, so "broadening the scope" is a common phrase.
* Perspective: This refers to a way of viewing something, so "broadening one's perspective" means expanding their understanding.
* Knowledge: "Broadening one's knowledge" means learning more about a topic or subject.
* Horizons: This is often used figuratively to mean expanding one's understanding or experiences.
Ultimately, the best noun for "broaden" depends on what you are trying to say.