General terms:
* Restriction: This is the most direct noun form, simply referring to something that limits or restricts.
* Limitation: This refers to the act of setting a limit or boundary.
* Constraint: This implies a strong limitation or restriction, often external.
* Regulation: This refers to a rule or guideline that restricts behavior.
More specific terms:
* Censorship: This refers to the suppression of speech or expression.
* Control: This implies a strong restriction or management of something.
* Bound: This suggests a limit or boundary that cannot be crossed.
* Threshold: This refers to a specific point beyond which something is restricted.
The best noun to use will depend on the specific situation and the nuances you want to convey. For example, "restriction" is a good general term, while "censorship" is more specific and implies a particular type of restriction on freedom of expression.