Does anybody know what a Brady letter is?

According to a previous Herald report, "'Brady letters' are used to alert criminal defendants that a police officer who might be called to testify against them has a sustained record of being untruthful in an official capacity and thus the credibility of the officer is called into serious question. The name is derived from a 1963 U.S. Supreme Court case, Brady v. Maryland, holding that the prosecution has a constitutional duty to disclose material evidence to the defense in criminal prosecutions. In succeeding cases, this duty was expanded to include impeachment evidence as well as exculpatory evidence. Failing to turn over this type of evidence can jeopardize the criminal cases as well as form basis for a civil rights lawsuit by the defendant."