Final answer:
An event that can make an otherwise valid agreement invalid is a contract defense, which is a legal argument or claim used to challenge the enforceability or validity of a contract. Examples of contract defenses include duress, fraud, mistake, and illegality.
Step-by-step explanation:
An event that can make an otherwise valid agreement invalid is a contract defense.
A contract defense is a legal argument or claim that can be used to challenge the enforceability or validity of a contract. There are various types of contract defenses, such as lack of capacity, fraud, duress, mistake, illegality, and unconscionability.
For example, if a person enters into a contract while under duress or coercion, they may argue that the agreement is invalid due to the presence of duress. Similarly, if a contract is based on fraudulent representations, the deceived party may assert a contract defense of fraud to render the agreement invalid.