Legal Definitions - collateral fact

A collateral fact is a fact that is not directly related to the issue in dispute, especially because it involves a different transaction from the one at issue. For example, in a case about a car accident, a collateral fact could be the fact that the defendant has a criminal record for a different offense. This fact is not directly related to the car accident, but it could be used to impeach the defendant's credibility as a witness.

Another example of a collateral fact could be a witness's opinion about a different matter that is not relevant to the case at hand. This opinion is not directly related to the issue in dispute, but it could be used to show the witness's bias or lack of credibility.