Waiver is a voluntary relinquishment or surrender of a known right or privilege. It may be verbal, in writing, or triggered by a person's own actions. For instance, waiver of personal jurisdiction is triggered if the defendant appears in the court's jurisdiction without first objecting to personal jurisdiction.
The doctrine of waiver plays an important role in an adverserial criminal justice system. A defendant may waive an issue or right either affirmatively or passively. The adverserial system assumes that the criminal defense lawyer always has the defendant's best interests in mind. Therefore, if a defense attorney fails to object to otherwise inadmissible evidence, lower courts will assume the defense attorney did so for strategic reasons. In this situation, a failure to preserve an objection can be converted into a passive waiver of the issue by the defendant.
The doctrine of waiver plays a crucial role in appeals because most issues are only appealable if they have been preserved for review in the lower court record.